I came across this expression at random, and when reading its definition and reading it in within context, it struck me as a particularly American thing to say. When trying to confirm my suspicion, I couldn't find this kind of information on its origin (merely that it formed in the 18th century and is the merging of two other words). I tried looking on the Merriam Webster site and Cambridge Dictionary (online) and didn't find an origin section that'd specify in which continent or country this expression was first used. Anyone knowledgeable?
The earliest use of jam-packed I have been able to find occurs in a review of a children’s program — the finale was an “emblematic representation of the majesty of Britannia” — in late 19th c. Cardiff:
I turned in last night at the Park-hall the place was jam-packed — a word specially coined for a special occasion, and the heat was suffocating. — “Observer,” “Notes” (column), Evening Express (Cardiff), 1 Dec. 1892.
When an author wishes to indicate a new coinage, the usual choice is simply to bracket it with quotation marks, especially if its meaning is obvious. A long appositive such as this one suggests jam-packed is still warm from the oven.
The first attestation in an American source occurs in a short story syndicated to a number of newspapers — not by an American, but by the popular Scottish author Samuel Rutherford Crocket:
“Jam-packed it was at ony rate when Marget an’ Archie got there. They were squeezed like herrings, and it was as warm as lying on your back in the pit an’ howking at the roof. — Samuel Rutherford Crocket, “That Popish Parson Fellow” [short story], The Times (Richmond VA), 24 Nov. 1895.
Written in the Lowland Scots eye dialect for which Crocket was noted, the story would be the last place one would look for an Americanism. The story also appeared in February 1896 in the Pocket Magazine and finally in a collection of Crocket’s short stories, Love Idylls, published in London in 1901. It is perhaps this publication that gave the Online Etymological Dictionary or its source a 1901 date for the first appearance of the word.
Whatever the case, the Scotsman Crocket and the anonymous Cardiff “Observer” prohibit otherwise straight linguistic lines from North America to the United Kingdom which the sightings of jam-packed in American sources after the turn of the century would suggest. After these two uses, British sources seem to go silent until the end of the 1910s.
In the year Crocket’s story collection went to press, however, a bit of filler appeared in at least two American publications:
Don't crowd your advertisements with matter and make them hard to read. A plain, readable, inviting advertisement is like a seat in the orchestra circle; a jam-packed, illegible one like space in the peanut gallery. — Current Advertising, v. 9 (Jan.-June 1901), The American Stationer v. 49, No. 1, 5 Jan. 1901.
While jam-packed doesn’t become especially frequent in the US until the mid-1920s and later, its use is spreading across the country:
“Ho!” rejoins William, “you ain't like a princess! … your hair's red, red! An' your eyes are kind o' green, they are! An' you're just jam- packed full o' freckles! — Josephine Dodge Daskam [Bacon], “The Heart of a Child” (short story), 1902.
— Advertisement, Cranbrook Herald (BC, Canada),12 Jan. 1905
And Silverstein's hall was just jam-packed. — Eleanor Gates, — Cupid, the Cow-Punch,1907
“Well, take the Perryville church, for instance. There's one that has some life to it! Ice Cream Socials and Strawberry Festivals and Pie Apple Parties and Turkey Dinners and Oyster Bay Clubs and Leap Year Leagues and what not, and what not! They gave one of these here Country Schools last fall, and the church was jam packed to the doors!” — The Lutheran Companion (Rock Island IL), v. 20, No. 11, 16 March 1912.
If the word appears in an official publication of the Evangelical Lutheran Augustana Synod of North America, overwhelmingly composed of Swedish immigrants and their descendants in the Midwest, then jam-packed has truly arrived in American English.
What stands out in these examples is that except for the filler about advertising and the ad itself, jam-packed appears in works of fiction imitating, as in Crocket, everyday, informal speech. The speaker in the church publication is merely a slangy straw man invented to be set to rights by an older Swedish Lutheran that social events are not the main mission of the church.
The first appearance in an Australian newspaper, at least among those digitized by the National Library of Australia, occurs in a Melbourne paper:
The Department has been assiduously pointing to its receipts on that afternoon, to prove that there was no extraordinary tram traffic, and just as persistently ignoring the plain fact that huge numbers of fares were uncollected on the jam-packed trams. — “In the Harbour City,” Punch (Melbourne), 6 May 1915.
And in 1919 Northern Ireland, a headline proclaims
Jam-packed Factories. Yet There is Little for the Public. Unheeded Official Advise. — Belfast Telegraph_ 30 Aug. 1919. BNA (paywall).
Across the United Kingdom, in the 1940s, jam-packed has 37 hits in the British Newspaper Archives (BNA) especially in articles about American films, most likely echoing Hollywood publicity. Each decade until 1980 has over a hundred hits, the 80s only 69, but 1,167 in the 1990s. Despite Crocket, there are no other Scottish sources from his time and thereafter that would suggest that jam-packed was particularly common in Scotland.
So...British or American?
Your question concerns the first attestation of jam-packed. That was answered by the first few paragraphs of my answer: Cardiff and a Scottish author publishing in American newspapers. Given the limits of what from this period is digitized and what is not, and that beyond its use in advertising, jam-packed was a slang expression not likely to appear often in print, evidence for a clear line across the Atlantic, however, is at best ambiguous.
While it is hardly difficult to draw linguistic lines from Scotland to Northern Ireland, one might expect the BNA to contain a few Scottish newspapers that use the expression at the turn of the century. Perhaps someone with access to the Scottich National Library digital holdings might have better luck.
It is, however, difficult to imagine that Crocket, despite his popularity in America, is the source of the word in North America. Jammed and packed were a common collocation:
To pass from Broadway, with its empty stages and almost deserted sidewalks, into the crowded, heated, jammed, packed Bell and Everett club room, was something like paddling one’s canoe out of a quiet trout stream into the roar and crash of Niagara… The New York Herald (NYC), 7 Nov. 1860.
Jam followed by a participial phrase headed by packed was also not uncommon:
Look at them ram jam, packed in like sardines in a box, or pins in their papers, stiﬂing with heat, panting for breath, listening eagerly and attentively to the impassioned utterances of a hired speaker. — America, a journal for Americans v.1, Apr.–Sept. 1888.
In other words, it is not outside the realm of possibility that jam-packed arose independently on both sides of the Atlantic without initial influence one way or the other. More pre-1900 attestations might add clarity, but these, if they exist, are not currently available online.
In moderate to rapid informal speech, to avoid the “reset” necessary to pronounce the following p, the d following the collocation jammed packed would likely be elided, as in red and purple. The result would be jam packed, which may ultimately be the independent source for both Crocket and the American usage. All jam, packed would require is a bit of reparsing and dropping the rhyming reduplication.
According to the OED, the adverbial and adjectival use of jam is U.S. in origin, attested from the early 19th century. I can't remember the last time I've heard jam-packed outside of a radio commercial or a movie depiction of a carnival barker, but I don't have good information on its prevalence or distribution.
The OED's etymological trail starts with the onomatopoeic verb jam meaning to force, squeeze, press, crush, stick, thrust, etc. something into a confined space. Something which is in close contact with or right up to something is thus jam up to it (or just jam), though this sounds somewhat old-fashioned or dialectical to me.
Also they had poll watchers from both sides in those days who sat jam up close to the ballot box with a list of voters on a clipboard. — from Ferrol Sams' Down Town (2008)
If something jam up is right up to or next to something, like a container being completely full because its contents rise to the brim, jam takes on the meaning of being thorough or excellent, thence jam-full for being completely filled.
1835 David Crockett An Account of Col. Crockett's Tour 192. [Andrew Jackson] went jam up for war; but the cabinet got him down to half heat. 1858 Samuel A. Hammett Piney Woods Tavern xiv. 146. The regular stage was jam full, and there was an extra put on, and that was jam full, and a leetle more.
Jam-packed is just a novel variation on jam-full, meaning tightly packed. The OED's earliest written attestation is from Ring Lardner, an American sportswriter, in 1925
Liberty 28 Mar. 5/1. This place is jam-packed Saturdays, from four o'clock on.
but a search on Google Books turns up earlier examples, like a line from The Madness of Philip and Other Tales of Childhood by Josephine Dodge Daskam, published 1902:
You don't look like the ones you tell about, anyway! Why… your hair's red, red! An' your eyes are kind o'green, they are! An' you're just jam-packed full o' freckles!
The verb form jam-pack is thus a back-formation from jam-packed.
Early instances of 'jam, packed' in U.S. newspapers
An Elephind newspaper database search yields a number of instances of "jam packed" in U.S. newspapers dating back to the 1880s. Following are eight unique instances spanning the period from December 25, 1883, through January 9, 1892.
From William Stoddard, "A Perfect Christmas" in the [Terre Haute, Indiana] Express (December 25, 1883):
The way they laughed about it gave him a great deal of courage, and he never cried when they took him by his red little hands, one on each side, and walked him into the house.
"Jane," said grandmother, what will we do with him? The house'll be choke, jam, packed full, and there isn't an extra bed."
This Christmas story reappeared in subsequent years in the Bloomington [Indiana] Progress (December 24, 1884), the Clare County [Michigan] Press (January 2, 1885), the Lake City [Colorado] Mining Register (January 2, 1885), and the Springfield [Ohio] Globe-Republic (January 11, 1885).
From an advertisement for Owen Pixley & Co. in the Springfield [Ohio] Globe-Republic (January 3, 1885):
The Fine Fancy Bows that ordinarily bring 25c, our way makes 10c; black, 6c. For a quarter you have your choice in a large deep case choke, jam, packed full, and they are not of the grab-bag quality either. Really fine little dressy ties may be selected from this case.
This instance is striking because the advertisement appears just one week before the same newspaper published the "Perfect Christmas" short story, which uses the same striking phrasing: "choke, jam, packed full."
From "Vive La Avalon: The Gallant Knights Entertain Their Guests in Royal Style: A Large Attendance at the Entertainment and Dance—An Interesting Programme Well Rendered," in the Santa Cruz [California] Sentinel (January 13, 1886):
The visitors and members then adjourned to Bernheim's Hall, which was cram, jam, packed full. All the wealth, beauty and intelligence of the city were there. The hall was never before so elaborately decorated. Overhead strings of bunting formed a canopy of bright colors, relieved here and there by the frescoed figures on the ceiling.
Here the series of adjectives is slightly altered from the version that appeared in the earlier instances, albeit only slightly: "choke, jam, packed full" has given way to "cram, jam, packed full."
And from "Private Opinion Made Public," in the Crawfordsville [Indiana] Daily Journal (January 9, 1892):
Lillian Lewis: "I ought to have had a better audience than I had last night. My company are better than ever before and should have filled the hall to overflowing. I won't come back any more if I can't get better audiences. Yes, sir, that house ought to have been jam packed last, night, so it ought."
Here, at last, we have "jam packed" as a stand-alone modifier, without "choke," "cram," or "full" as companion modifiers. It doesn't seem far-fetched to speculate that the shorter form arose out of the longer one—especially given the geographical location of many of these early instances in a fairly compact area of the U.S. Midwest—Indiana (three times) Ohio (twice), and Michigan—with outlier instances farther west in Colorado and California.
All eight instances cited thus far in this answer—four of them unique and four reprints—appeared in print before the earliest instance of the "jam-packed" cited in KarlG's answer, from the Cardiff [Wales] Evening Express of December 1, 1892.
Early instances of 'jammed, packed' in U.S. newspapers and periodicals
There is, however, a second and even earlier line of modifiers of the form 'jammed, packed" in U.S. newspapers, dating to the late 1840s. KarlG's answer notes one example of this phrasing from the New York Herald (November 7, 1860) and another from the monthly journal America (1888). Here are twelve additional instances of the wording, from the years 1848 through 1885.
From "Cleveland [Ohio] Newspaper Digest Jan. 1 to Dec. 31, 1848," in Annals of Cleveland, volume 31, years 1848:
Sept. 28; ed: 2/1 - "At an early hour on Tuesday evening, (Sept. 26), the Court House was filled, crammed, jammed, packed with persons wishing to hear from the mouth of our revered Representative, Mr. Giddings, the position he occupies the reason therefore.
From "Local Matters," in the [Petersburg, Virginia] Daily Express (October 26, 1855):
Octagon Hall was not only filled, but jammed, packed — crowded. Once in, there was no going hence, but all we could do was simply to stand and gaze on the beautiful specimens of handiwork presented in such rich array on every hand.
From "Letter from High Private," in the Houston [Texas] Tri-Weekly Telegraph (November 8, 1865):
This town [Galveston] is just overrun with strangers. "That's what's the matter." Every hotel and boarding house is filled with them, and even the private dwellings are accommodating hundreds of them. Foxes have holes and the birds of the air have nests, but the son of man hath not where to lay his head, especially if he be a "son of a gun" and hath not friends. There is not in this town a house, barn, stable, shop, store or church "for rent." All are rammed, jammed, packed, until even their sides ache with continued pressure. Two dollars an hour is charged fur the privilege of leaning against a post. and three dollars for a block to sit on.
From "The Schutzenfest: Third Day," in the Charleston [South Carolina] Daily News (May 9, 1868):
In the dancing hall the fun was huge and orderly. It was crowded, jammed, packed with merry dancers. Gentlemen with the emblematic white ribbon in their button-holes flitted to and fro, and the ladies, young and old, danced with a grace and persistency worthy of their name.
From "The University," in the [St Paul, Minnesota] Daily Globe (June 2, 1882):
Before 9 o'clock in the morning the chapel of the university presented a brilliant and animated scene. Ushers were dashing to and fro, bringing in chairs and seating the spectators. At 9 o'clock the chapel was filled, crowded, Jammed—packed full. Every aisle was full of chairs, the stage was crowded.
From "Washington Letter," in the New Ulm [Minnesota] Weekly Review (March 7, 1883):
The week has been characterized by interesting events, among which may be mentioned the international bench-show, the first dog show ever held in Washington, successful operatic engagement the city has ever known. Society changed its "at home's" to "at opera's" and attended en masse. Patti came, sang and conquered, her last appearance being honored by a house crammed, jammed, packed and wedged to such an intense extent that the authorities took the precaution of having a strong fire force stationed at the nearest available point to the theatre, with a man at each nozzle, and the water turned on.
From an advertisement for Haverly's Minstrels, in the New York Clipper (December 22, 1883):
THE PROSPEROUS BLACKBIRDS OF A NATION SINGING THEIR SONGS IN THE SUNNY SOUTH AND MEETING WITH JAMMED, PACKED, OVERFLOWING HOUSES NIGHTLY. THE FIFTH MINSTREL COMPANY THROUGH DIXIE THIS SEASON, AND ALL THE PEOPLE PROCLAIM US THE MIGHTIEST, THE BRIGHTEST, THE WITTIEST AND THE BEST.
From "Bloomington Markets," in the [Bloomington, Illinois] Weekly Pantagraph (February 15, 1884):
Roll butter should be nicely wrapped, each roll separate, in cloth wet in weak brine, and the box should be lined with paper and the butter carefully and tightly (not jammed) packed In the box.
From "Chips," in the [Salt Lake City, Utah] Salt Lake Herald (February 26, 1884):
A jammed, packed and uncomfortable house greeted Mrs Clayton's appearance at the Theatre on Sunday evening.
From an untitled item in the [Pueblo] Colorado Daily Chieftain (March 25, 1884):
In reply as to whether he had purchased extensively he said that he had brought three times as many goods as heretofore, but did not know where they could put them, as the store was jammed, packed full.
From "The Biggest and Best of Them All," in the [Astoria, Oregon] Daily Morning Astorian (August 14, 1885):
At every performance the huge canvas tents of John Robinson's great Three Ring Circuses, were literally jammed, packed and crammed with happy, laughing people, in many instances the employes of large business houses, being sent by their employers, who purchased the tickets in bodies of a hundred or more.
And from "Illinois" in the New York Clipper (October 24, 1885):
Hooley's Theatre.—Crowded, jammed, packed houses every night. Louis Aldrich is on now with "In His Power." Next week, Sidney Rosenfeld's Opera Co.
These twelve instances cover a large geographical area of the United States, ranging New York, Virginia, South Carolina, Ohio, Illinois, Minnesota, Texas, Colorado, Utah, and Oregon.
The profusion of instances of "jammed, packed" and later "jam, packed"—often as part of longer phrases that included choke or crammed and (in many cases) terminated with full—in U.S. publications from 1848 through 1892 lends considerable weight to the suggestion that jam-packed as a stand-alone modifier arose out of these earlier forms. Under the circumstances, I think that the case for U.S. origin of jam-packed is quite strong.