Green’s Dictionary of Slang

knockout drops n.

also knock-drops, knockout, knockout pills, k.o. drops, k.o.d., the drop

1. [late 19C+] chloral hydrate mixed into a drink to render an innocent victim unconscious.

2. [late 19C+] a soothing linctus, usu. based on laudanum or opium, used to soothe fractious young children.

3. [1900s] in fig. use, something powerful, destructive.

4. [late 1910s+] (US) drugged or adulterated liquor.