Longman Dictionary of Contemporary English - knockout

~1 n 1 an act of knocking your opponent down in boxing so that he cannot get up again  (The fight ended in a knockout.) 2 knockout punch/blow a) a hard hit that knocks someone down so that they cannot get up again  (- see also knock out knock1) b) an action or event that causes defeat or failure  (High interest rates have been a knockout blow to the business.) 3 informal someone or something that is very attractive or successful  (Baby, you're a knockout.) 4 BrE a competition in which winning players or teams continue playing until there is only one winner ~2 adj informal making someone unconscious  (knockout pills | the knockout punch)