What is a mickey of liquor?

A "mickey" or "Mickey Finn" is a knockout drink. It can refer to a deliberately strong drink or one tainted with a chemical agent to cause unconsciousness.

The name comes from an infamous bartender (Mickey Finn) who dosed unsuspecting clients with chloral hydrate, then stole their cash and valuables. The victims would awaken with no recollection of how they were robbed.