Earliest occurrences of 'dude' in its modern slang sense
The earliest citation for dude in J.E. Lighter, Random House Historical Dictionary of American Slang (1994) is from 1877 in Frederic Remington, Selected Letters 15:
Don't send me any more [drawings of] women or any more dudes.
A longer excerpt from the cited letter (written by Remington to his friend Scott Turner) appears in the 1988 edition of the Selected Letters [combined snippets]:
[Highland Military Academy]
[1877]
... I hope you will excuse the blots I got on the upper end of this sheet. They don't mean anything in particular, but I wish you would make some similar ones on your return letter. Draw me a good picture, only one, and I'll be your slave forever. Give us battle between the Russians and Turks, or Indians and soldiers ...
... Don't send me any more women or any more dudes. Send me Indians, cowboys, villans or toughs. These are what I want. ...
Lighter offers this definition of dude:
dude n. {orig. unkn.} 1.a. a usu. over-refined or effete man or boy who is pretentiously concerned with his clothes, grooming, manners, etc.; dandy; (broadly) West. a city person, esp. if new to the West; a guest at a DUDE RANCH. Now S[tandard] E[nglish] ... b. Mil. a soldier newly inducted or arrived [first citation from 1936] ... c. a foolish or obnoxious fellow [first citation from 1967] ... 2.a. a male person; fellow [first citation from 1883] ... b. (used in direct address to a male person; MAN) [first citation from "1877–88] ... c. usu. pl. a person of either sex [first citation from 1974]" ... d. Army. a German soldier [first citation from 1918] ... 3. a fancy or excellent example [first citation from 1919] ... 4. item; thing. [first citation from 1960]
That, at any rate is Lighter's report on the evolution of the word dude. The unfortunately inexact reference to 1877–88 for the earliest use of dude in sense 2(b) reflects the uncertainty of the date when the particular (unidentified) play, by Jack Crawford (who died in 1917), containing the reference was published. Here is the portion of dialogue where it appears, in Jack Crawford, Three Plays (1966) [snippet view]:
Jack: What you want Bill?
Sam: Ugh. Me an scalp.
Jack: Oh, you want his scalp? Why redskin dude, is there any other little thing you'd like? Don't be modest.
Sam: Ugh. Where's Bill?
The second volume of J.S. Farmer & W.E. Henley, Slang & Its Analogues (1891) has a much shorter definition of dude:
DUDE, *-** (American).—A swell; fop; 'masher.' ... {From Scots DUDS = clothes; [citation to an 1870 occurrence of dud omitted].} Derivatives are DUDETTE and DUDINETTE = a young girl affecting the airs of a belle; DUDINE = a female masher.
The earliest citation in Farmer & Henley to an occurrence of the word dude is from 1883. Farmer & Henley's definition of masher, by the way, is amusingly precise:
MASHER, subs. (common).—1. ... A species of Don Juan in a small way of business: specifically among choristers and actresses. Hence (2) a dandy.
The mushrooming consciousness of 'dude' in the popular press
Notwithstanding Frederic Remington's earlier usage of the term in a private letter, national awareness of "dude" was remarkably sudden and explosive. The trigger appears to taken the form of a "New York Letter in Boston Advertiser." Though the Library of Congress's Chronicling America database of historical newspapers evidently does not include a copy of the relevant Advertiser, it does include newspapers across the country—the Omaha [Nebraska] Daily Bee (March 2, 1883), the Washington [D.C.] Evening Star (March 3, 1883), the Dallas Daily Herald (March 4, 1883), and the Fort Worth [Texas] Daily Gazette (March 7, 1883)—that reprinted the item. It begins:
Late advices from your city announce that you have no "dudes" there. Do you kno what a "dude" is? The name was just given, I think, in one of our daily papers, to a curious specimen of the genus homo which has lately appeared in New York. He is young, thin, pale, often hatchet-faced, almost always narrow-chested and small limbed. His extremely tight trousers painfully accentuate his lack of figure. ...
Within a month, newspapers from Wheeling, West Virginia to Milan, Tennessee to Austin, Texas to Hailey, Idaho to Astoria, Oregon to Sacramento, California were reporting the latest details about the habits of dudes the the New York press could gin up, reprinting satirical poems about dudes, and speculating about how soon local folk could expect to catch a glimpse of this ludicrous figure.
Speculation as to the word's origin was rampant. A article titled "The Development of the Dude," in The Nation (March 8, 1883), offered this origin theory:
When a foreign term is suddenly naturalized we may be sure that there is something in the atmosphere of the place of adoption which makes it convenient and useful. "Dude" is said to be originally a London music hall term, but it has been transplanted here, and its constant use shows that it is for some reason well fitted to take a permanent place in the vocabulary of fashion.
A couple of weeks later, the Brooklyn Eagle—as reported by, among other papers the Daily [Astoria, Oregon] Astorian of March 28, 1883— took a more cautious approach to the question of the term's origin:
The New York correspondent of the Brooklyn Eagle, a sort of "Man About Town," notes the introduction of a new word into the language. It is d-u-d-e or d-o-o-d, the spelling not having been distinctly settled yet. No body knows where the word came from, but it has sprung into popularity within the past few weeks, and everybody is using it.
And the New York Graphic—reprinted by, among others, the Iola [Kansas] Register 0f March 30, 1883—weighed in with this view:
Where the "dude" got his name nobody knows. The dictionaries throw no light on it. The most reasonable theory appears to be that it was given to him by Columbia College students—among some of the classes of which he is said to be quite numerous—in consequence of his languidly drawling out the French "Je-doute" when appealed to o behalf of any positive statement That the "dude" is of collegiate origin appears certain. He is in reality but a copy of the English undergraduate who aspires to be considered a superior intellectual being.
Conclusions
Use of dude in the slang sense of "man or boy excessively interested in his mode of dress" may go back to 1877, but the wonder year for the term was clearly 1883, when various New York newspapers—and one national weekly magazine—seized on the term and wrote about the character it supposedly identified—and newspapers all over the United States picked up the the story (and the slang) and ran with it. Perhaps the closest similar term, as far as sudden journalistic interest and subsequent massive popular adoption go, in the past fifty years is yuppie, although preppy comes close. In both cases a largely unknown slang term burst without warning onto the national scene, becoming the subject of countless sociological analyses and hostile comedic put-downs.
The range of meanings that dude has had over the past 130 years is nicely summarized in the Random House Historical Dictionary of American Slang (1994), but its precise origins remain unsettled. Merriam-Webster's Eleventh Collegiate Dictionary (2003), for example, assigns dude the "origin unknown" label.