Early Occurrences of 'Bunk'
Potentially the earliest relevant match for bunk that a Google Books search turns up is as part of a compound word, slaw-bunk. From Gideon Hawley, Rev. Gideon Hawley’s Journey to Oghquaga [later Windsor, New York], 1753 (July 31, 1794):
Mr. Woodbridge and I with assiduity pursued our way, one after the other, through bushes, and sloughs, water and mire as our guide directed. And at dusk we arrived at the nearest houses between fort Hunter and Schoharry ; but did not put up until we came to what was accounted a publick house, but very unfit for the entertainment of gentlemen strangers. It had only one room. In that room was what is called a slaw-bunk, with a straw-bed, on which we lodged.
The account of Hawley’s journey appears in a letter composed in 1794, but the day-by-day specificity of his account suggests that he was working from a journal or diary written at the time of the journey (1753). Unfortunately, it's impossible to say whether Hawley was familiar with the term “slaw-bunk” in 1753 or learned of it afterward and added it to his account in 1794.
A Google Books search also turns up several early instances of bunk as a stand-alone word. From George Washington, “General Orders, Wednesday, March 8, 1780,” in The Writings of George Washington from the Original Manuscript Sources, 1745–1799, volume 18:
The hot season approaching, every possible attention is to be paid to cleanliness in the interior and environs of the camp; Sinks are to be dug without delay. Every fair day the windows and doors of all the huts should be kept open the greatest part of the day, and the beding straw and bunks frequently aired.
From James Tilton, “Observations on the Yellow Fever, as it appeared at Wilmington (Delaware) in the Summer and Autumn of 1798” (June 18, 1799), in The Medical Repository, volume 3 (1800):
When the Yellow Fever rages, the principles which I have adopted would direct every human creature, both sick and well (a necessary watch or guard excepted), to quit the infected city or town, and encamp on the open plains. The sick should carefully avoid plank floors to their tents, and lie upon cots or bunks, with- their legs standing upon a fresh surface of the ground, or a grass sod.
From “Death of the Indian Prophet,” dated August 23, 1815, in The American Magazine (September 1815):
About thirteen years ago, while lighting his pipe, he [“one of the Chiefs of the Alleghanies”] fell back upon his bunk, upon which he was then sitting, and continued in a state of insensibility for six or eight hours ; his family supposing him dead, had made preparations for laying him out, and, while in the act of removing him from his bunk, he revived. His first words were, “don’t be alarmed, I have seen Heaven ; call the nations together, that I may tell them what I have seen and heard.”
From James Mann, Medical Sketches of the Campaigns of 1812, 13, 14 (1816):
Each patient should be provided with a separate bed in a moveable bunk. When bunks are attached to the walls of a room, as has been sometimes the mode, they are not easily cleansed. The proper dimensions of a bunk are 6 feet 8 inches in length, and 2 feet 8 inches in breadth.
During the campaign of 1813, I furnished myself with one of these bed-bottoms, upon which, lodging was easier and much cooler than on a sack of straw in a bunk of wood.
From “Letter from the Secretary of War Transmitting a System of Field Service & Police, and a System of Martial Law, for the Government of the Army of the United States” (December 22, 1819):
The name of each soldier will be labeled on his bunk, in the place the most apparent, and the number which he bears in his squad, placed against his firelock and accouterments. ... The knapsack of each man will be placed on the lower shelf, at the foot of his bunk, ready to be slung and charged with his effects, excepting such as are habitually in use.
From Israel Potter, The Life and Remarkable Adventures of Israel Potter (1824):
These "ready furnished rooms" [in London] were nothing but miserable apartments in garrets, and contain but few more conveniences than what many of our common prisons in America afford — a bunk of straw, with two or three old blankets, a couple of chairs, and a rough table about three feet square, with an article or two of iron in which to cook our victuals...
Perhaps the most telling thing about these instances of bunk or bunks—all of which are from U.S. sources—is that none of the writers considers it necessary to define those terms. Evidently, the word is so well established in the setting where it is used that no explanation of its meaning is deemed necessary.
Early Occurrences of 'Bunker'
Meanwhile, a Google Book search for bunker yields a number of early matches—this time concentrated in Scottish writing. From Allan Ramsay, “Christ’s Kirk on the Green,” canto 3 (by 1724, but not earlier than 1718), in Miscellaneous Works of That Celebrated Scotch Poet, Allan Ramsay (1724):
Sae whiles they toolied, whiles they drank,/Till a' their Sense was smor'd;/And in their Maws there was nae Mank;/Upon the Furms some snor'd:/Ithers frae aff the bunkers sank,/Wi' Een like Collops scor'd;/Some ramm'd their Nodles wi' a Clank,/E’en like a thick scul’d lord/On Posts that day.
The editor of the 1776 edition of Ramsay’s poems includes the following note:
Bunkers, a bench or sort of long low chests, that serve for seats
From John Sinclair, Observations on the Scottish Dialect (1782):
A bunker. A window-seat.
From John Jamieson, An Etymological Dictionary of the Scottish Language (1818):
BUNKER, BUNKART, s. 1. A bench or sort of low chest serving for a seat. 2. A seat in a window, which also serves for a chest, opening with a hinged lid. 3. It seems to be the same word which is used to denote an earthen seat in the fields, Aberd[een].
From Robert Burns, “Tam O’ Shanter” (written in 1791), collected in Matthew G. Lewis, ed., Tales of Wonder (1805):
A winnock-bunker in the east,/There sat auld Nick, in shape of beast;/A towzie-tyke, black, grim, and large,/To give them music was his charge:/He screw’d the pipes and gart them skirl,/Till roof and rafters a’ did dirl.
In a footnote in this edition, Lewis says that “winnock-bunker” means “a window.” But in his book English Romantic Poetry and Prose (1956), Russell Noyes says that the term means “window-seat.”
From Walter Scott, “The Heart of Mid-Lothian,” in Tales of My Landlord (1818):
But so it was, that although there were better farm-houses on the land than Woodend, and certainly much prettier girls than Jeanie Deans, yet it did somehow befal that the blank in the Laird's time was not so pleasantly filled up as it had been. There was no seat accommodated him so well as the "bunker" at Woodend, and no face he loved so much to gaze on as Jeanie Deans's.
How the Early Occurrences Match Up
To sum up the Google Books data, the first occurrence of bunker or bunkers (in the sense of benches or chests) appears in a Scottish poem by Allan Ramsay in 1724, followed by other mentions in 1782, 1791 (in Burns’s poem “Tam O’ Shanter”), and 1818 (twice). The first occurrence of bunk may date to 1753, as part of the term slaw-bunk; but the first clear-cut match in the Google Books results is from George Washington’s general order of March 8, 1780. Ramsay’s use of bunkers antedates the earliest OED citation for bunker by 34 years and the earliest MW citation by 115 years. Washington’s use of bunks antedates the earliest OED citation for bunk by 35 years, but is 27 years later than the earliest MW citation. However, Hawley’s use of slaw-bunk—if it came from his contemporaneous journal of his travels—antedates the earliest MW citation by 5 years.
The question remains: Did bunker and bunk arise independently of one another, or did bunk evolve from bunker? John Russell Bartlett, Dictionary of Americanisms (1848) reports both the American usage for bunk and the Scottish usage for bunker, but doesn’t argue that one came from the other:
BUNK. (Ang Sax. benc. Germ. bank. Danish baenk, a bench, a form.) A wooden case used in country taverns and in offices which serves alike for a seat during the day and for a bed at night. They are common throughout the Northern States.
Dr. Jamieson has the word bunker, a bench, or sort of low chests that serve for seats—also, a seat in a window, which serves for a chest, opening with a hinged lid.
Nevertheless the similarity between the two words at this stage in their development is striking.