Nothing. Shoot OFF your mouth, however, means to say something ill-considered, usually something rude or aggressive. It's as if your mouth were a gun and you shot it without thinking.

Derived from the 1800's probably in America because Americans love guns. Like me for example.

Shoot your mouth off, shoot the breeze, let's shoot pool, the star just shot by me, and "Don't shoot!" are just a few common phrases containing the words 'shoot' and 'shot'.

Rather than exaxt sentences, you need to look at the context to understand if someone was "shooting off their mouth". The boy spoke rudely to the teacher, but he slipped with a swear word that got him detention.

The man complained about the check point, shooting off his mouth at the police officer, but the man's breathalizer registered 2.0.

My friend always shoots her mouth off that I don't pick nice boyfriends, until a loser she dated dumped her.

They shoot water from their mouth.