watch one's mouth

verb2

1. •
   To be careful about one's speech, especially with regard to disrespectful or profane language.

   • You really need to watch your mouth. You're a teacher now!
2. •
   In the imperative form, used as a warning to choose one's words carefully or else be silent, for fear of consequences.

   • Hey. Watch your mouth. Don't you realize who's listening?