A regional street term for a depressant, see there
Bogart was knocked out with a Mickey Fin in the Maltese Falcon (1941)
(1) A mixture of whiskey with chloral hydrate, the latter of which causes hypotension, pinpoint pupils, cardiac arrhythmia and, in high doses, gastric irritability, perforation, hepatotoxicity, and nephrotoxicity
(2) Croton oil and alcohol
Mickey Finn was barkeeper of the Lone Star Saloon & Palm Garden on Chicago’s Whiskey Row in the 1890s, who used his concoction to incapacitate clients and relieve them of their worldly goods. A Mickey Finn was used in The Maltese Falcon to produce the so-called “knock-out drop” effect on Humphrey Bogart, an effect which was not seen on volunteers in a study of mid-1970s vintage. Chlorals are the oldest class of hypnotics; trichlorethanol is the first metabolite of chloral and causes its hypnotic effect.
Reference Clin Pharm & Therap 1972; 13:50