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1. slang A particularly heavy blow; a powerful punch able or intended to knock someone out. Creed connected a Sunday punch to Tyson's head early in the third round, sending him sprawling to the mat. She may look scrawny, but you don't want to be on the receiving end of one of her Sunday punches.
2. slang By extension, any maneuver, action, or remark that causes devastating harm to one's opponent. She dredged up her opponent's tax records during the debate, apparently convinced it was a Sunday punch that would bury him. The rebel forces were preparing to throw a Sunday punch at the fascists in command of the country.
n. a very solid and destructive blow; one’s best blow. Ralph aimed a Sunday punch at Frederick’s nose, but he missed and spun about, striking his elbow on the banister.