Meaning of Sunday punch

Sun'day punch'

Pronunciation: [key]
  1. the most powerful and effective punch of a boxer, esp. the punch used in trying to gain a knockout.
  2. anything, as an armed force, line of argument, etc., capable of inflicting a powerful blow on a hostile or opposing person or thing.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.