When someone slips a powdered drug or purgative into a drink to render its
drinker unconscious or otherwise helpless, he has concocted a Mickey Finn
or Mickey. The term seems to have originated in Chicago in the late
nineteenth century and has been attributed to an underworld figure named
Mickey Finn, remembered for his sleight of hand but nothing else.
Possibly, Mickey Finn derives from some unknown bartender named Mickey,
whose potent mixes quickly finished his formerly belecose patrons, but one
guess is as good as another here. The original Mickey Finn is said to have
been a laxative commonly used for horses.