Watch Your Mouth!
Be careful what you say.

It would be difficult to literally watch one's mouth, but watch also means to control or to keep it out of trouble - "I need to watch my kids" means I need to take care of them. Here, watch your mouth means to keep it under control and therefore to be careful of what you say. This is said to someone who has said something bad or profane or said to someone as a warning.

"The boss is here, we'll have to watch our mouths."

"Damn it, I forgot my homework!"

"Watch your mouth!"