What is knockout? What is knockout? The act of knocking out.The state of being knocked out.A blow that knocks out an opponent. Sports A victory in boxing in which one's opponent is unable to rise from the canvas within a specified time after being knocked down or is judged too injured to continue.Sports The act of winning a boxing match in this way: won the fight by a knockout. Slang A strikingly attractive or impressive person or thing.Capable of knocking out: a knockout punch. Strikingly attractive or impressive.Having a specific single gene removed from the genome by genetic manipulation: knockout mice used in an experiment.