knockout drops

from the GNU version of the Collaborative International Dictionary of English.

• noun plural Slang, U. S. Drops of any drug, used to a person to fall asleep or become unconscious or stupefied for the purpose of robbery, etc.; they are usually mixed into a drink so that the person consuming it does so unknowingly. One compound used for this purpose is chloral hydrate.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun chloral hydrate in combination with alcohol; usually administered surreptitiously to make the drinker unconscious