knockout drops n.
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.