From Longman Dictionary of Contemporary English(you) watch your mouth(you) watch your mouthspoken informal used to tell someone not to speak in such a rude way → mouth
Examples from the Corpus
(you) watch your mouth• She knew he had spoken, she had watched his mouth move.• It went up 32 points Thursday, a day Newt watched his mouth.• I remember watching her mouth while she talked.• We have to watch our mouths and let our kids know that bad words are unacceptable. 