knockout drops

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Slang A solution, usually of chloral hydrate in alcohol, put into a drink surreptitiously in order to render the drinker unconscious.

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. chloral hydrate in combination with alcohol; usually administered surreptitiously to make the drinker unconscious