What it means
Mouth off means running your gob, giving cheek, or talking tough like you’re untouchable. It’s that loud backchat aimed at someone you should probably pipe down around, bouncers, teachers, bosses, even your mate’s mum. Usually you’re trying to look hard for an audience, and it ends with you getting shut down, chucked out, or told to pack it in sooner than you think.
Usage examples
"Gaz started mouthing off at the bouncer about the dress code, calling him a jobsworth. Two minutes later he’s outside, kebab in hand, acting like he won anyway."
"He started mouthing off about the referee from the stands and got himself escorted out before half time."
Where it comes from
To mouth off is to run your gob loudly, giving cheek, boasting or shouting your opinion where it is not wanted. The mouth does the work, off it goes without a filter. Said of the loudmouth at the bar, the kid talking back, the keyboard warrior. All volume, little thought, and usually heading for trouble.
Other ways to say it
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