The Collaborative International Dictionary of English v.0.48:

knockout drops \knockout drops\, knock-out drops \knock-out
drops\n. pl.
   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. [Slang, U. S.]
   [Webster 1913 Suppl. + PJC]

WordNet (r) 3.0 (2006):

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