Green’s Dictionary of Slang

1. complete, authentic.

1902		Ade Girl Proposition 1: He thought he had seen some 24-carat Tizums when he had attended College and hung around the Fem Sem, but the Girl that he now beheld was in a class by herself.
1918		H.V. O’Brien diary 7 Mar. in Wine, Women and War (1926) 41: Drool by Col. Y—. A 22 blank, they say. Spends his time chasing waitresses.
1927		H.C. Witwer Classics in Sl. 3: The semifinal brought together a couple more twenty-eight carat bums in ‘Kid’ Edwards and ‘One Round’ McTague.
1938		D. Runyon ‘Cemetery Bait’ in Runyon on Broadway (1954) 521: A 22-carat fink, a fink being a character who is lower than a mudcat’s vest pocket.
1952		J. Jones From Here to Eternity (1998) 790: It was, all of it, as near to being a full-fledged 24-carat civilian as any thirty-year-man ever could get.
1961		G.L. Coon Meanwhile, Back at the Front (1962) 94: You’re a real forty-eight-carat son of a bitch, aren’t you?
1969		P. Boyle All Looks Yellow to the Jaundiced Eye 53: A shockproof, fully automatic, twenty-two-carat bum-boy.
1970		A. Sillitoe Start in Life (1979) 71: I’m a bastard, a real no-good, genuine twenty-two-carat bastard in every sense of the word.
1972		O. Hawkins Ghetto Sketches 55: Gimme eight bones ’n put it on. Twenty carat gin-u-wine Swiss!
1974		D. Ireland Burn 35: I got down on my knees, feeling like a twenty-four-carat idiot.
1983		‘Heat Moon’ Blue Highways 205: Why do you think she’s called Tiffany? Pure class. Twenty-four carat.
1989		(con. 1950s–60s) in G. Tremlett Little Legs 97: You are a twenty-two carat cunt.

2. very wealthy.

1902		Bulletin (Sydney) 18 Oct. 12/3: Despite all the talk about Melba and her old chums, her private entertaining was confined to the very Nicest. Three great dinners helped to make things merry and delightful for immediate relatives and a few 22-carat guests.
1958		T. Capote Breakfast at Tiffany’s 38: Holly [...] caught me reading: Miss Holly Golightly, of the Boston Golightlys, making every day a holiday for the 24-carat Rusty Trawler.

3. totally reliable, wholly trustworthy.

1962

R. Cook Crust on its Uppers 45: You come out twenty-two carat. [Ibid.] 47: I tell you, they’re absolutely twenty-two.