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+] drugged or adulterated liquor.