Green’s Dictionary of Slang

1. chloral hydrate mixed into a drink to render an innocent victim unconscious.

1892		Eve. World (NY) 12 July 3/5: Several gangs that made a business of drugging and robbing immigrants have been caught [...] The police were enabled to arraign the ‘man catchers’ captured with the ‘knockout’ drops in their possession.
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.
1895		J.S. Wood Yale Yarns 152: Our dandy team played a logy, tired sort of game, as if each man had been given knock-out drops.
1898		F. Norris Moran of the Lady Letty 41: Mee dear friend Jim put a knock-me-out drop into your Manhattan cocktail.
1902		E. Townsend Chimmie Fadden and Mr Paul 101: Put all de sugar and milk dat ’s coming to me in me foist cup [...] for dat ’s de only part of tea dat is n’t knock-out drops to me.
1909		C.B. Chrysler White Slavery 39: The Slaver slips her the K.O.D, knockout drops or drugs.
1909		I.L. Nascher Wretches of Povertyville 60: Knock-out drops is a solution of chloral hydrate, the pungent taste of which is immediately noticed by a person in his sober senses [...] Knock-out drops are supplied in half dram vials, each vial containing 20 grains of chloral, a vial full being used at a time.
1910		F.P. Dunne Mr Dooley Says 105: Th’ sons iv Mars [...] can get annything fr’m sulphuric acid to knock-out dhrops.
1914		Jackson & Hellyer Vocab. Criminal Sl. 64: Amongst gamblers and badgers a ‘peter’ is a sleeping potion, a ‘knockout,’ such as hydrate of chloral.
1920		A. Baer No Thumb Prints 6 Nov. [synd. col.] A bottle of K.O. drops haven’t much effect on a whole ocean.
1921		P. & T. Casey Gay-cat 270: He had been given ‘knockout drops’ and been robbed of his paper of identification.
1927		E. Wallace Squeaker (1950) 165: Sutton had opened the champagne, had filled and emptied one glass. In the other he had carefully dropped thirty drops of a water-like fluid from the little brown phial [...] In the course of his chequered career he had twice used ‘the drop’.
1936		K. Mackenzie Living Rough 110: I took him up to an hotel room and slipped some K.O. drops in his moonshine.
1938		X. Herbert Capricornia (1939) 234: Go on — get out — or I’ll get ’em to put knock-drops in your grog.
1939		Wodehouse Uncle Fred in the Springtime 268: ‘When he was running that club of his, it was only by a judicious use of knock-out drops that he was able to preserve order and harmony’ [ibid.] 303: ‘I still don’t see [...] why he should have slipped kayo drops in — ’.
1940		S. Lister Mistral Hotel (1951) 106: What did you slip in his drink? Knock-out drops?
1952		(con. 1920s) ‘Harry Grey’ Hoods (1953) 223: I gave them some knockout drops in their whiskey.
1966		J. Mills Panic in Needle Park (1971) 123: With chlorohydrate. No. I didn’t use it. I mean I didn’t take it. I used it for knockout drops. It comes in crystals and you just put a few drops in somebody’s drink and they get very sleepy.
1988		J. Healy Grass Arena (1990) 106: More came running, held me, got my trousers down, shoved a needle full of knock-out drops in me.
1991		(con. 1920s) O.D. Brooks Legs 209: Before he was supposed to leave they put knock-out drops in his drink and took off with everything he had.

2. a soothing linctus, usu. based on laudanum or opium, used to soothe fractious young children.

1984

Partridge DSUE (8th edn) 656: late C.19–20.

3. in fig. use, something powerful, destructive.

1908

N.Y. Times 30 May in Fleming Unforgettable Season (1981) 81: He [...] unloaded the choicest collection of knockout drops dealt since Amos Rusie was doing a specialty.

4. (US) drugged or adulterated liquor.

1931

A. Hardin ‘Volstead English’ in AS VII:2 87: Terms used for intoxicating liquor: Knock-out drops.