knock dead - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.