Green’s Dictionary of Slang

a euph. for damn adv., i.e. very or very much.

1789		N. Webster Dissertations on the Eng. Lang. 385: The word (dern) is in common use in New England and pronounced darn. It has not, however, the sense it had formerly; it is now used as an adverb to qualify an adjective, as darn sweet; denoting a great degree of the quality.
c.1858		J.H. Green Reformed Gambler 133: It’s tarnal well for you, that I aint at hum, for if I was, I’d give it to ye, darn quick, tew.
1869		Elk Co. Advocate 20 Nov. 3/1: If he succeeds in killing deer as well as he does ion printing a good paper he will do dern well.
1895		E. Pugh Street in Suburbia 43: A right-darn good ’un.
1898		F. Dumont Dumont’s Joke Book 26: I’ll bore a hole through your Yankee carkiss darn quick.
1901		B. Pain De Omnibus 134: Ah, she were a wrong un – a reg’lar right-darn wrong un!
1903		Meade Co. News (KS) 28 May 1/5: They know dern well they’ve struck a place / That’s full of great bananzas.
1905		E.W. Hornung A Thief in the Night (1992) 381: I should judge [...] that it’s for Mr Maguire to say, or not to say, just as he darn pleases.
1908		N.Y. Tribune 26 July 26/2: Your deal, Jim, [...] but I’d like darn well to cut the deck.
1914		E. Dyson Spats’ Fact’ry (1922) 64: I ain’t give up nothin’, an’ I darn well won’t give up nothin’.
1916		Day Book (Chicago) 12 May 25/2: Optimism is the feeling that hardluckis never going to come to you, though you know darn well that it will.
1917		G. Herriman ‘Baron Bean’ [comic strip] And dern nice I calls it of you too.
1920		E. O’Neill Beyond the Horizon I i: I suppose it’s that year in college gave you a liking for that kind of stuff. I’m darn glad I stopped with High School.
1921		P. & T. Casey Gay-cat 206: I thinks immedjit o’ the Frisco Kid an’ his dern yeller cur.
1922		Wash. Times (DC) 21 May 31/5: An acrostic [...] is a man who does not believe in nothing and lives up to it darn well.
1922		Seattle Star (WA) 27 Nov. 3/4: I’d like dern well to chuck it in the Sound.
1930		W.R. Burnett Iron Man 278: ‘We ain’t friend no more.’ [...] ‘You ought to be darn glad you’re not.’.
1934		E. Caldwell Tobacco Road (1958) 13: I’m getting pretty durn tired of it.
1943		G. Kerouac letter 24 Mar. in Charters I (1995) 50: Well, I thought that was darn decent of Uncle Sam and now I hope everything will turn out O.K.
1944		C. Himes ‘Money Don’t Spend in the Stir’ in Coll. Stories (1990) 197: ‘You durn right it’s mine,’ I yells.
1949		A.B. Hollingshead Elmtown’s Youth (2007) 288: Some of them are so darn dumb.
1950		‘Neville Shute’ Town Like Alice 84: I’ll make darn sure there’s something crook with the truck.
1956		N. Algren Walk on the Wild Side 116: So dern hongry if I went out in the sun I’d be prostrayted like a dog.
1957		R. Prather Always Leave ’Em Dying 106: I was so worried, so darn worried.
1961		T. Taylor Baron’s Court All Change (2011) 25: ‘I couldn’t care less about anything. Life’s too darn short’.
1979		Fantastic Four Annual 8: It darn near got us Blackbeard’s treasure.
1999		Guardian G2 10 Aug. 22: I darn near died.