A dirty deed or a trick.
If you find drugs in your beer, you'd say "Damn! Someone slipped me a Mickey Finn!"
A dose of knockout drops, such as chloral hydrate, a sedative/hypnotic, usually administered to some hapless barfly as a preamble to rolling him.
They slipped him a Mickey Finn, and when he woke up the next day, his wallet, wristwatch and jewelry were gone.