Green’s Dictionary of Slang

(US) a euph. for damn! excl.

1781		Pennsylvania Journal 20 June n.p.: In New England prophane swearing [...] is so far from polite as to be criminal, and many [use] substitutions such as darn it, for d-n it [OED].
1833		C.A. Davis Letters of Major J. Downing (1835) 44: ‘Why, darn it,’ says I, ‘Mr. Van Buren, I’m glad to hear that.’.
1840		Southern Literary Messenger VI 508/2: ‘O, darn it to darnation,’ muttered Joe [DA].
1848		J.R. Lowell Biglow Papers (1880) 89: Ware’s my left hand? O, darn it, yes, I recollect wut’s come on ’t.
1869		H.B. Stowe Oldtown Folks 392: Under great pressure of provocation Sam Lawson freely said, ‘Darn it!’.
1873		W.H. Thomes Slaver’s Adventures 47: Darn it, man [...] can’t you render a little assistance to a friend in distress?
1890		H. Nisbet ‘Bail Up!’ 6: Darn it all!
1890		Sporting Times 1 Feb. 1/5: On a clear winter day, drifting slowly away / With a hole in her hose. (She said, ‘darn it!’).
1907		S.E. White Arizona Nights 38: Darn! six hours to day!
1914		S. Lewis Our Mr Wrenn (1936) 76: Oh, darn it all. I feel rotten. I wish I was dead!
1915		Van Loan ‘The Extra Man’ Buck Parvin 20: Darn it! [...] I wasn’t crawling.
1936		L. Dent ‘Angelfish’ in Goulart (1967) 224: ‘Darn it!’ she said.
1951		Cusack & James Come in Spinner (1960) 211: Darn it all.
1961		M. Braly Felony Tank (1962) 121: Darn it, Dottie, put some clothes on.
1969		Aus. Women’s Wkly 30 Apr. 67/1: No, darn it, it is not a cold.
1978		P. Theroux Picture Palace 292: Underneath the glamour and achievement one was a very simple soul, saying ‘Golly’ and ‘Darn it’ and doting on cheese and biscuits.
2000		P. Cornwell Last Precinct 69: Darn. I guess this will be all right, but not for the salad dressing.