Be quiet or cease talking immediately. Can be rude, aggressive, or jocular depending on the context. Shut your mouth, Bobby—no one asked for the opinion of a dork like you! A: "So, where do you think we can sell these stolen goods?" B: "Shut your mouth! Don't you have enough sense not to talk about that in public?" A: "Wow, that Cadillac has seen better days. How much did you pay for that little gem?" B: "Ah, shut your mouth, Dave. I think it looks just fine."