shoot off mouth
shoot off (one's) mouth
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 off his mouth 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 off your mouth 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 off his mouth about how far he can throw a football.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
shoot off (one's) mouth/face Slang
1. To speak indiscreetly.
2. To brag; boast.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.