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Jack is a name that only losers or super smug people have. "Jack shut up" is what you yell when Jack is being a loud, annoying, corny pain in the butt.
Jack: "I’m gonna be a rock star!" Me: "Jack shut up."
Jack: "I’m the best at everything." Me: "Jack shut up."
Jack: "I just won a gold medal." Me: "Jack shut up."