Definition from Wiktionary, the free dictionary
- (idiomatic) the mouth of someone who talks too much, especially by making exaggerated claims or by inappropriately revealing information
- Shut your big mouth or I'll shut it for you.
- (idiomatic) a person who has a "big mouth" (sense #1)
- Shut up, big mouth!
Usage notes 
- Numerous common phrases involve one's "big mouth" or "mouth" being shut or open.