keep (one's) mouth shut
To not discuss or mention something. He needs to keep his mouth shut and stop telling our family secrets to everyone in town! A: "It wasn't my fault. Brian said&mdash" B: "Keep your mouth shut, Chad!"
keep one's mouth shut
Be quiet; don't reveal confidential information. For example, The teachers told us to keep our mouths shut during the entire presentation, or You can't tell Carol anything; she's incapable of keeping her mouth shut. Also see hold one's tongue.