Knockout/ˈnɑːkaʊt/ Definition 1 of 5Pro Tip 1/2Complimentary Usage Used to express admiration, often in a romantic or physical attractiveness context. Upon entering the room, everyone agreed she was a knockout. Pro Tip 2/2Slang Expression Informal and found mostly in casual conversation rather than formal writing. You should have seen him at the party, he looked like a knockout.