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.