saying smart ass things and generally
bitching at a fast pace. Yelling out paragraphs of bitchyness really quickly. Blabbing nonsensical things in an argument just to take up time and have your voice
heard.
(the running part means that is is
done at a fast pace or else it would be called "walking your mouth")
"they were in a rational argument and she said something that offended him, so he started running his mouth to show his disaproval"
EX.
Him: hey,
Jessica
Her: My name isn't fucking
Jessica
Him: fuck, fuck, fuck, man, fuck, just fuck it man, fuck, fuck, fuck
Her: What the hell is wrong with you!?
Him: you need to stop running your mouth.
Her: Uh, I'm pretty
sure it's you who is running their mouth.