watch your mouth/tongue

watch (one's) mouth

To be very mindful or cautious about what one says; to control what one says in order to avoid offensive language. Watch your mouth! I don't care how you speak at home, that's not how we talk here. After being threatened with detention for my snide remarks in class, I knew I had to watch my mouth for the rest of the semester.
See also: mouth, watch

watch your ˈmouth/ˈtongue

be careful what you say in order not to offend somebody or make them angry: Now, you just watch your mouth around your grandparents, Billy! OPPOSITE: shoot your mouth off (about something)
See also: mouth, tongue, watch