informal if you have a big mouth, you talk too much, especially about things that should be secret.
Helen's got such a big mouth - the news'll be all over the town by tonight.I knew I shouldn't have mentioned the letter. Oh dear, me and my big mouth!
a big mouth
1. A tendency or habit of speaking incessantly and indiscreetly, especially about sensitive or secret information. If I'd known you had such a big mouth, I'd have never shared my secret with you!God you've got a big mouth! Don't you ever stop talking about other people's business?
2. A person who talks in such a manner. I can't stand Terry's new husband, he is such a big mouth!
big mouth
1. n. a person who talks too much or too loudly; someone who tells secrets. (Also a term of address.) Okay, big mouth! Shut up!
2. tv. to spread secrets around. Why do you always have to big mouth everything around?