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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.
Greatly amuse, astonish, or thrill someone, as in This new song will knock them dead. This slangy hyperbolic expression was first recorded in 1889. Also see knock the socks off.
1. To kill with a blow.
2. Slang To affect strongly and positively: a performance that knocked the audience dead.