Watch your mouth

this verbal warning and boundary is provided when someone takes personal offense to what someone else is saying. It is meant to urge the person to reconsider what they are saying.

The term can be followed up with an “or else” but it is usually the final warning before a consequence is given (a slap, getting blocked, etc.)

Watch out for danger behind your back (like you might get jumped or something).

Watch your back.

Keep an extra set of eyes on the back of your head.

The thing stupid people say to someone that curses. Referenced in one of jacks film's videos

To watch your behind, used in aviation as a warning by your wingmen.

Watch what you're saying bro

T. slang that means relax and shut up

synonym for "nize it"

Watch your back.
Keep an eye on what could be behind you.
Turn around and look.

This phrase simply means that you should watch what you are saying because the person saying this to you will most likely knock your teeth out of your oral cavity because they are not liking what you are saying.