The Mickey Finn is most likely named for the manager and bartender of a Chicago establishment, the Lone Star Saloon and Palm Garden Restaurant, which operated from 1896 to 1903 in the city’s South Loop neighborhood on South State Street. In December 1903, several Chicago newspapers document that a Michael “Mickey” Finn managed the Lone Star Saloon and was accused of using knockout drops to incapacitate and rob some of his customers.
A Mickey Finn (or simply Mickey) is a slang term for a drink laced with a drug (especially chloral hydrate) given to someone without their knowledge in order to incapacitate them. Serving someone a Mickey Finn is most commonly referred to as slipping a mickey.
Figuratively it means that someone is trying to dupe someone else.