Green’s Dictionary of Slang

1. first-class.

1881		‘Bill Nye’ Bill Nye and Boomerang 286: The gorgeous eighteen-karat-stem-winding profanity of the present day.
1902		Bulletin (Sydney) 20 Dec. 17/1: No dog, by the way, will eat bandicoot, raw or cooked, though it is considered 18-carat by bush epicures.
1902		Mills & Scott [perf. George Robey] ‘The Artist’ 🎵 I’m living in a garret, my style is eighteen carat.
1903		Tacoma Times (WA) 28 Dec. 3/5: The 18-Karat gumshoe stars of the New York detective force are frothing at the mouth.
1914		Joyce ‘Ivy Day in the Committee Room’ Dubliners (1956) 122: His father was a decent, respectable man [...] But I’m greatly afraid our friend is not nineteen carat.
1919		Perrysburg Jrnl (Wood Co., OH) 22 May 2/2: You’ll know you’ve got a real man [...] the 18-K article.
1928		J. Callahan Man’s Grim Justice 48: This is a real eighteen-carat pipe [...] it’ll be a walkaway.
1936		Monks & Finklehoffe Brother Rat I i: The smoothest fourteen-carat number that ever hit the Shenandoah valley.
1965		J.P. Carstairs Concrete Kimono 15: I’m a fourteen-carat whore.

2. absolute, complete.

1902		‘O. Henry’ ‘The Marionettes’ in Rolling Stones (1913) 79: I don’t think — I ever met — such an — eighteen-carat rascal as you are, Doctor.
1920		H.C. Witwer Kid Scanlon 100: Any burg that’s got a couple of sure enough eighteen-carat boobs in it, known to the trade as suckers, has got a chance.
1932		V.F. Nelson Prison Days and Nights 93: [He] was known among the recipients generally as ‘an 18-karat sucker’.
1940		W.R. Burnett High Sierra in Four Novels (1984) 377: Of all the fourteen-carat saps! Starting out a caper with a woman and a dog.
1953		T. Runyon In For Life 139: I wanted to believe practically all guards were seventeen-carat bastards.
1962		(con. WWII) J.O. Killens And Then We Heard The Thunder (1964) 80: He was a sixteen-karat phony and he needed her to believe he was the real thing.
1967		(con. 1950s) McAleer & Dickson Unit Pride (1981) 336: You’re a fourteen-carat dirty bastard, Coggins.
2000		R.N. Mitra A Very Insipid Passion 21: He had made a genuine eighteen-carat suicide attempt.
2003		Weinberg et al. 100 Dastardly Little Detective Stories 188: ‘Why,’ Giles says, and his voice is even in the dark, ‘you’d be makin’ a eighteen-carat mistake.’.