Green’s Dictionary of Slang

1. stupefying or liable to cause unconsciousness, orig. of a drug.

1895		E. Townsend Chimmie Fadden Explains 11: If Mr. Burton wasn’t good t’ Miss Fannie I’d put a knock-out pill in his cocktail.
1909		R. Service ‘Lost’ in Ballads of a Cheechako 133: And there is the blizzard waiting to give me a knockout blow.
1920		‘Doss Chiderdoss’ ‘The Reformed Pub’ Sporting Times 17 July 1/3: If what they’re advocatin’ happens, and bars get a knock-out biff, there won’t be no pubs at all, but somethin’ absolutely diff.
1930		L. Lower Here’s Luck 105: If all other methods failed, I could give him some knock-out drops with his supper.
1947		(con. 1944) J.H. Burns Gallery (1948) 89: He was given a knockout drug for paranoia and delusions of persecution.
1966		T. Keyes All Night Stand 122: Knock-out stuff, but with the hash it sort of picks you up.
1973		E. Droge Patrolman 93: The [...] doctor-in-charge reluctantly agreed to a 50cc. shot of a knockout drug.
1985		G. Norman in Norman (1921) 122: It ended up with Powell letting dad have a supply of ‘knock-out’ pills.

2. excellent, wonderful, the very best.

1908		H. Green Maison De Shine 240: It’d be a knockout screech.
1920		F.S. Fitzgerald This Side of Paradise in Bodley Head Scott Fitzgerald III (1960) 161: At any rate we’ll have really knock-out rooms.
1932		(con. 1910s) J.T. Farrell Young Lonigan in Studs Lonigan (1936) 83: He laughed to himself, thinking [...] what a knockout story it would be for Dan and the guys.
1943		Baker ‘Influence of American Sl. on Australia’ in AS XVIII:4 256: A knock-out sketch of a hot baby and an ace-high sport got beat up by some tough eggs before the cops woke up. It was a dirty meal.
1951		Cusack & James Come in Spinner (1960) 300: I want to have the most absolutely knockout wedding.
1966		T. Keyes All Night Stand 162: Jean Wenchet, who I think is a knock-out chick in anybody’s book.
1977		Rolling Stone 22 Sept. 73: [advert by A&W Publishers, Inc.] You’ll find 700 absolutely knock-out record jackets in this eye-popping new book!
1987		(con. 1967) E. Spencer Welcome to Vietnam (1989) 25: I go by helicopter to Khe Sanh. What a knock-out view.
1999		Guardian Weekend 5 June 34: The really knock-out thing about Catholics [...] is we eat God.

3. very attractive.

1949		N. Algren Man with the Golden Arm 190: This is the slickest little knockout broad in seventeen states.
1974		D. Mamet Sexual Perversity in Chicago (1994) 97: That’s all it takes ... to make the difference between a knockout looking broad, and a nothing looking broad.
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 27: Then Tim sees this knockout girl [...] and he thinks he’s going to get some.
1991		B. McCarthy Vice Cop 26: I see this knock-out girl coming in.
2000		Indep. Rev. 7 Apr. 1: My sister, I should say, is an absolute knock-out beauty.
2018		I. Welsh Dead Man’s Trousers 87: Vicky is wearing a knockout glittering black dress.