Green’s Dictionary of Slang

1. (orig. US) a knockout drug, poss. chloral hydrate, mixed into an unsuspecting victim’s drink; thus as v.

1903		Inter Ocean (Chicago) 17 Dec. 3/1: ‘Mickey’ Finn’s Lone Star saloon was lonely last night and his palm garden was not palmy. Finn’s ‘knock-out’ drops are charged with ‘knocking him out’ of business.
1918		Wash. Post 23 June 1: Mr. Hoyne had a report that waiters used a certain powder in the dishes of known opponents to the system. The powders, according to Mr. Hoyne, produced nausea and were known as ‘Mickey Finns.’ It is thought that many cases of supposed ptomaine poisoning reported after meals in downtown cafes and hotels may have been caused by the ‘Mickey Finns.’.
1923		O.O. McIntyre New York Day by Day 2 Sept. [synd. col.] In that period when Tom Sharkey’s saloon, Maison Doree, Sans Souci [...] dotted Fourteenth street, the ‘knock-out drop’ or ‘Mickey Finn’ claimed almost a dozen victims a night.
1924		T.A. Dorgan in Zwilling TAD Lex. (1993) 57: Why don’t that big slob get on an excursion boat — He don’t wanna fish he wants a sail — Wish I had a drink and a Mike Finn for him.
1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 124/1: mickey flynn or mickey finn. A knockout dose (often cigar ashes in a carbonated drink) administered to an addict or a sucker.
1938		(ref. to 1920s) R. McAlmon Being Geniuses Together 298: I told Jimmie to mix a cocktail to make the man sick. ‘You know, Jimmie, a real micky-fine.’.
1939		Wodehouse Uncle Fred in the Springtime 275: ‘Impostor A has just laid Baxter out cold with a knock-out drop [...] Impostor A has just slipped Baxter a Mickey Finn’.
1946		Mezzrow & Wolfe Really the Blues 294: For the first time in my life I had met up with a great old American institution, the Mickey Finn.
1956		B. Behan Quare Fellow (1960) Act I: Many’s the seaman myself and Meena gave the hey and a do, and Mickey Finn to.
1959		Mad mag. Mar. 30: Fenwick Furd passes out after downing a ’Mickey Finn’.
1960		C. Himes Big Gold Dream 140: I ain’t no fighter. And I has to have some kind of way to protect myself. So I just carrys me a little Mickey Finn.
1971		Wodehouse Much Obliged, Jeeves 129: Jeeves [...] gave him a Mickey Finn.
1992		Randolph & Legman Ozark Folksongs and Folklore I 173: Some male hanger-on or minor client was presumably given a ‘mickey finn’ of chloral hydrate in his drink.
2004		J. Ellroy ‘Jungletown Jihad’ in Destination: Morgue! (2004) 359: The motherfuckers Mickey Finned me.
2004		E. Weiner Drop Dead, My Lovely (2005) 91: The odd pharmaceutical surprises administered via hypo, herbal tea and Mickey Finn.
2018		‘Aidan Truhen’ Price You Pay 99: [S]he put a Mickey Finn in your cold milk or your Chardonnay.
2021		J. Ellroy Widespread Panic 200: They Mickey Finn’d Marymount girls and got in their pants.

2. attrib. use of sense 1; note joc. nonce use in cit. 1915.

1915

T.A. Dorgan in Zwilling TAD Lex. (1993) 57: {Sign in saloon:} Try a Michael Finneka Cocktail.

3. (US drugs) any form of depressant or tranquilizer.

1938		Pittsburgh Courier (PA) 27 Aug. 11/1: His voice must had some kind of [...] Mickey Finn in it because it seemed to help you nix your nervousness.
2001		ONDCP Street Terms 15: Mickey Finn — Depressants.
2005		E. Weiner Big Boat to Bye-Bye 185: ‘It wasn’t me. I took a Mickey finn’ [...] ‘That trank story?’ He sneered.

4. (Aus.) in carnival use, the person who goes out and touts for business.

1941

K. Tennant Battlers 141: In the show world a ‘gee-man’ or ‘micky finn’ was socially on the level of a duck’s feet. He is the man who goes out in the crowd and touts for custom.

5. (N.Z. prison) a sleeping pill.

1982		G. Newbold Big Huey 251: mickey finn (n) Sleeping pill.
1999		D. Looser ‘Boob Jargon’ in NZEJ 13 33: mickey finn n. A sleeping pill - ‘slip them the Mickey’.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 116/1: Mickey (also Mickey Finn or Mickey Flynn) n. 1 a sleeping tablet.