Green’s Dictionary of Slang

a general intensifier, the most extreme, the best, the biggest.

1850		W.K. Northall Life and Recollections of Yankee Hill 193: And don’t I go home and tell the old folk; and when they come home, don’t the old folk kick up the darndest row?
1851		‘How Sally Hooter Got Snake-Bit’ in T.A. Burke Polly Peablossom’s Wedding 67: ‘That Yazoo,’ said Mike, ‘is the durndest hole that ever came along.’.
1858		Jeffersonian (Stroudsburg PA) 4 Mar. 1/3: I’ve got one of the darnedest black caps you ever seed.
1866		C.H. Smith Bill Arp 18: I tried my darn’dst yesterday to disperse and retire, but it was no go.
1867		Leeds Times 8 June 6/1: The Greasers gave us the durnesdest whipping at Santa Rosa.
1876		‘Dan de Quille’ Big Bonanza (1947) 285: He’s jist the durndest dog out!
1886		Cheyenne Transporter (Darlington, Indian Terr.) 12 Aug. 3/2: Blame me if them ain’t the darnedest beans I ever seen!
1889		Mower Co. Transcript (Lansing, MN) 17 Apr. 3/3: Bates whittled out [...] the duredest lookin’ jumpin’-jack you ever seed.
1898		Ranche & Range (N. Yakima, WA) 13 Aug. 10/3: There’s the darnedest mistake I ever seen in a high class newspaper.
1902		Laurens Advertiser (SC) 15 Jan. 4/1: Brother Harwood [...] lemme beg your pardon the durnedest worst way. I had no idee —.
1919		E. Ferber ‘Un Morso doo Pang’ One Basket (1947) 78: French. It’s the darnedest language!
1926		Harry Reser’s Six Jumping Jacks ‘Out in the New Mown Hay’ 🎵 He told her the darndest lies, / Out in the new mown hay.
1930		G. Milburn ‘Down in the Mohawk Valley’ Hobo’s Hornbook 50: For Nell had the durndest notions ever spawned in human heads.
1938		C. Beaton Cecil Beaton’s N.Y. 93: They think up the darndest things.
1942		L.A. Times 13 Sept. 107/4: ‘Honest, Dad [...] you do get the darnedest — ’.
1959		A. Zugsmith Beat Generation 86: The darnedest things you don’t notice.
1966		C. Stead Cotters’ England (1980) 283: He’s the darndest flirt I ever met. The playboy of the Western World.

(orig. US) to do one’s very best, occas. one’s worst, but always one’s utmost.

1837		R.M. Bird Nick of the Woods I 182: Here’s for you, you everlasting varmints – due you darndest!
1889		Western Kansas World 19 Oct. 1/6: I’m having the gosh-durnedest best time in my life.
1890		Sun (NY) 7 Dec. 24/1: A room in which a human being can sleep with a charocal stove doing its durnedest is not [etc.].
1900		Bulletin (Sydney) 15 Dec. 24/4: Judged by the principle that a newspaper gets the sort of letters it wants, Sydney Daily Telegraph is doing its ‘darnedest’ to advocate the claims of N.S.W. Premier Lyne to be called on to form the first Federal Ministry.
1905		Lincoln Co. Leader (Toledo, OR) 3 Mar. 4/1: Senator fulton is undoubtedly doing his darnedest.
1912		Bulletin (Sydney) 1 Aug. 28/2: Give it its due, South ‘did its darnedest,’ but its darnedest wasn’t darned enough; and Eastern Suburbs, sadly bedraggled and mud-stained, climbed out pantingly with 10 points to South’s seven.
1913		J. London Valley of the Moon (1914) 324: I did my dangdest, too, not to make a sound.
1917		Wodehouse ‘At Geisenheimer’s’ Man with Two Left Feet 113: He was there to do his durnedest.
1922		Wash. Herald 24 Feb. 1/3: Grandpop Rockefeller [...] will do his darnedest to entertain and interest her.
1926		Sun. Post 21 Feb. 13/3: The bold little bustle is douing its ‘durnedest’ toget back into favour [...] It has got itself worn by several weary mannequins.
1935		G. Ingram Cockney Cavalcade 278: And to show you, I’ll do my durnedest.
1937		Eve. Teleg. (Dundee) 29 Nov. 8/4: It was a good, lively, earnest game [with] every man [...] doing his durnedest.