Green’s Dictionary of Slang

1. anything exceptional, superlative.

1920		T.H. Kelly What Outfit, Buddy? 22: At a distance you think these darn French villages are the cat’s knee-knuckles, so to speak, but when you get in them it’s the same old stuff.
1922		Appleton Post-Crescent (WI) 3 May 7/4: frog’s eyebrows – Nice fine.
1923		Wodehouse Leave it to Psmith (1993) 493: Well, if this ain’t the cat’s whiskers!
1925		Chillicothe (MO) Constitution 9 Dec. 2/2: ‘Blubber a bibful’ or ‘crocodile’s adenoids’ are slang phrases that are offensive and condemned to die.
1926		Wood & Goddard Dict. Amer. Sl. 10: cat’s whiskers, cat’s pajamas, cat’s meaow [...] etc. Something excellent.
1929		H.W. Brecht Downfall 73: Gee, Dick, that’s the cat’s.
1935		C. Odets Awake and Sing! II ii: You put one over on that little shrimp. The cat’s whiskers, Mom?
1939		‘Flann O’Brien’ At Swim-Two-Birds 31: That was the pig’s whiskers. That was funny all right.
1940		H. Drake-Brockman Hot Gold I ii: don: How’s the beer? mac: The cat’s whiskers.
1952		H. Boyle Associated Press 30 June n.p.: A teenager has to keep up on his slang. At the moment something that used to be known as the cat’s whiskers is now called ‘sly,’ ‘really neat,’ ‘the real George,’ or ‘deadly boo’ [W&F].
1975		F.A. Cleary A Pocketful of Years 44: Susie thinks herself cat’s whiskers making big thing of a bit of pakeha blood in her veins.
1999		Indep. on Sun. Culture 1 Aug. 5: It’s the cat’s whiskers.
2003		Ken Greenwald WordWizard Inc. Feb. 🌐 Other sources list ‘[...] the bullfrog’s beard, the canary’s tusks.’.
2004		Guardian Science 13 May 7: This book is the cat’s whiskers.
2014		Observer Rev. 26 Jan. 17/3: Good? It’s the cat’s whiskers!

2. a superior person, or someone who poses as such.

1923		Wodehouse Leave it to Psmith (1993) 499: When it comes to doping out a scheme, you’re the snake’s eyebrows! [Ibid.] 517: When it comes to the smooth stuff, old girl, you’re the oyster’s eye-tooth!
1924		Hecht & Bodenheim Cutie 22: Before little Herman was nine years old, everybody knew he was the lily’s whiskers.
1936		N. Marsh Death in Ecstasy 122: Why, he thinks I’m the sandfly’s garters. [Ibid.] 211: Well, ain’t you the clam’s cuticle!
1949		(con. 1943–5) A. Murphy To Hell and Back (1950) 136: I wish you could see my sailor. He is the ‘cat’s whiskers.’.
1963		A. Baron Lowlife (2001) 130: Of course, you think you’re the cat’s whiskers, don’t you?
1982		P. Bailey An Eng. Madam 71: I thought Sam was my ideal man, I thought he was the cat’s whiskers.
2000		Guardian 28 Jan. 24: She was the cat’s whiskers.