The Collaborative International Dictionary
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.]
WordNet
n. chloral hydrate in combination with alcohol; usually administered surreptitiously to make the drinker unconscious [syn: Mickey Finn]
Wikipedia
Usage examples of "knockout drops".
I was going to get you a chewing tobacco can full of knockout drops from some big saloon in some place like Sheboygan.
We'll forget and forgive, we'll take them up to the cabin for a drink, slip them the knockout drops then ease them soft and gently over the side.
The knockout drops could have been in the bottom of that glass, unnoticed by any of us.
They lured me here under false pretenses, and then they gave me knockout drops so I couldn't get away overnight.
I rushed back to Judey, and we very carefully lifted the cheese and added five knockout drops to each piece of pizza.