knock somebody dead

knock (one) dead

To strongly impress one. Often used as an imperative to give encouragement. Good luck on the audition! Knock 'em dead, kid! Your design is going to knock the judges dead. They're going to swoon over it.
See also: dead, knock

knock somebody ˈdead

(spoken) impress somebody very much: You look fabulous — you’ll knock ’em dead tonight.
See also: dead, knock, somebody