1. A person who talks incessantly, indiscreetly, and/or in a noisy, boastful manner. That loud mouth Bill had better learn to stop discussing other peoples' business, or he's going to find himself with a lot of unwanted enemies. I can't stand Terry's new husband—he's such a loud mouth!

2. A tendency or habit of speaking in such a manner. That loud mouth of yours is going to get you in trouble one of these days. If I'd known you had such a loud mouth, I'd have never shared my secret with you!