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

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "knockout drops":
   Amytal, Amytal pill, Demerol, Dolophine, H, Luminal, Luminal pill,
   M, Mickey, Mickey Finn, Nembutal, Nembutal pill, Seconal,
   Seconal pill, Tuinal, Tuinal pill, alcohol, amobarbital sodium,
   analgesic, anesthetic, anodyne, aperitif, barb, barbiturate,
   barbiturate pill, black stuff, blue, blue angel, blue devil,
   blue heaven, blue velvet, calmative, chaser, chloral hydrate,
   cocktail, codeine, codeine cough syrup, depressant, depressor,
   doch-an-dorrach, dolly, dope, downer, drink, drug, eye-opener,
   goofball, hard stuff, heroin, highball, hop, horse, hypnotic, junk,
   knockout drop, laudanum, liquor, lotus, meperidine, methadone,
   mixed drink, morphia, morphine, narcotic, nightcap, opiate, opium,
   pacifier, pain killer, pain-killer, paregoric, parting cup,
   pen yan, phenobarbital, phenobarbital sodium, pousse-cafe, punch,
   purple heart, quietener, rainbow, red, scag, secobarbital sodium,
   sedative, shit, sleep-inducer, sleeper, sleeping draught,
   sleeping pill, smack, sodium thiopental, somnifacient, soother,
   soothing syrup, soporific, stirrup cup, sundowner, tar,
   tranquilizer, turps, wee doch-an-dorrach, white stuff, yellow,
   yellow jacket