A humdinger is a remarkable or outstanding person or thing.
The OED has it as originally US dating (as hum-dinger) from 1905, but says the origin is unknown.
Where does the word humdinger come from? Was it used before 1905?
Here is the entry for humdinger in Eric Partridge, A Dictionary of Slang and Unconventional English (1961):
humdinger. A fast aircraft or vehicle; a smooth-running engine: Services, but mostly R.A.F.: adopted in mid-1940 from American airmen. American s[ource]: echoic: hum (speed) + dinger (something forceful).
Partridge published a dictionary of RAF slang in 1945, so he's especially knowledgeable about words used in that service.
Wentworth & Flexner, Dictionary of American Slang (1961) addresses the dinger part of the term in its entry for humdinger:
humdinger n. Something or someone very remarkable or admirable. See dinger. c1905: archaic and dial. now. adj. Remarkable.
And here is part 1 of the dinger entry from that same source:
dinger n. 1 Any remarkable person or thing. Obs. See hum-dinger. 1951: "President Truman used it not once, but twice, in characterizing a sermon he had heard. ... 'That was a dinger, chaplain,' said [he], and a moment later: 'I said it was a dinger.'"
Chapman & Kipfer, Dictionary of American Slang, Third Edition (1996) removes its predecessor's comment that humdinger is "archaic and dial. now," though it reasserts the "by 1905" coinage period. I can't help wondering if Bob Dylan's rhyme in "I Shall Be Free" ("She's a humdinger/Folk singer/Dead ringer/For a thingamajigger"), which came out in 1963, helped bring humdinger back into popular usage. The Ngram Viewer graph for humdinger is not inconsistent with that possibility, though the relationship does seem rather far-fetched.
The third volume of the prodigious Farmer & Henley, Slang and Its Analogues, with alphabetical entries ranging from "Fla. to Hyps." was published in 1893 and has no entry for humdinger, so the term certainly wasn't in widespread use long before 1905.
A Google Books search, however, turns up Roberts et al. v. Date et al. (1903), a U.S. Circuit Court of Appeals decision involving interests in an Alaska mining claim called "the Humdinger" that had been filed by the defendants in the case on June 22, 1900. Other mining claims under dispute in the same case were called "the Marie," "the Discovery," and "the Eagle Canyon." The Humdinger claim was located on Slate creek, so "Humdinger" does not appear to be a place name (as opposed to a descriptive name).
Another early reference appears in the Paint, Oil and Drug Review (February 6, 1907):
A little incident that occurred in Indian Territory recently shows how the Indians are cheated. Some time since a well drilled by a Mr. "X" was reported as having been found to be dry. The rigging was removed, the derrick torn don,the hole capped and apparently the well was abandoned. A short time thereafter there appeared an advertisement offering for lease the 20 acres belonging to an Indian girl and adjoining the lease where Mr. "X" had found the "dry" hole. The lease was offered for sale by a master in chancery. It was sold for a cash bonus of $2,550, together with one-fourth royalty on all oil discovered. A representative of Mr. "X" was the purchaser. After the sale had been made to him Mr. "X" confided to a friend that the "dry" hole had not been quite so dry as reported. In fact, it was, in the parlance of the craft, a "humdinger."
Yet another early Google Books mention of humdinger comes from the records of the Brown Swiss Cattle Breeders' Association (1908), which details the parenting exploits of a bull named Humdinger that lived in Illinois and sired 18 registered calves that were born between January 31, 1906, and November 19, 1907. Humdinger himself was born on December 5, 1903, and registered (#2187) at some unspecified time thereafter.
I found a humdinger earlier than the OED's 1905. I'll quote a large section as it gives the full context of why the "rhetorical humdinger" under discussion "is one of the architectural wonders of word construction".
The Butte Inter Mountain, January 27, 1902:
REPRESENTATIVE SIBLEY'S SPEECH
If it were not for the fact that he is from the staid and conservative Keystone state, that has never encouraged wind-jamming politicians, it might be suspected that Representative Sibley was trying to make himself popular by using the brand of oratory that stampeded the democratic national convention in 1896.
When a man gets up in congress and says that the present accomplishment of irrigation will be "pressing a poisoned chalice to the lips of the farming classes of this nation," it is time for the people to look out for another boy orator.
But we fear that Mr. Sibley was so intent on springing his "poisoned chalice" smile on congress that he lost sight of any such trifling thing as truth in his speech. We admit that the metaphor, epigram, or whatever Mr. Sibley choose to call it, is one of the architectural wonders of word construction.
We have analyzed it from all sides -- full face, three-quarters and profile, and we can find no laws in it from the stand-point of the debating society. It is rhetorical humdinger that will doubtless be accepted as standard, and that will figure in many a country schoolhouse debate.
And Stephen Goranson found a 19th century humdinger:
In an ad for new cabs, of five levels of quality, the best being "A humdinger, only… [$]15.00"
Paper: Daily Register Gazette, published as The Rockford Daily Register-Gazette.; Date: 04-07-1896; Page: 2; col. 7 Location: Rockford, Illinois. [Amer. Hist. News.]
As Sven Yargs' answer shows, it seems likely humdinger is from the earlier dinger:
Any remarkable person or thing.
The OED has the noun dinger chronologically from 1809 and etymologically from the verb ding:
"3. fig. To ‘beat’, overcome, surpass, excel."
This ding is from [?a1513] or 1724.