shoot mouth off
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shoot (one's) mouth off
1. To speak without discretion; to speak too loudly or freely, especially about sensitive topics or information. We would have gotten away with our plan if your dumb cousin hadn't started shooting his mouth off all over town. I figured it went without saying that I didn't want to talk about my divorce at Daniel's wedding, but you just had to go and shoot your mouth off like that!
2. To be annoyingly or overbearingly talkative, especially in a bragging or boastful manner. There's some guy at the other end of the bar shooting his mouth off about how far he can throw a football.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
shoot one's mouth off
Inf. to boast or talk too much; to tell secrets. Don't pay any attention to Bob. He's always shooting his mouth off. Oh, Sally! Stop shooting off your mouth! You don't know what you're talking about.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.