Verb. To procrastinate or find unimportant activities with the purpose to simply annoy people for your own pleasure. Such activities include; aimlessly wondering around in someone's office, blablabla-ing with friends online while munching on food for the purpose of munching or otherwise focusing on being an nuisance.
Q: Honey why are you walking around in my office with a piece of KFC stuck in your mouth while blablabla-ing with your friends?
A: Oh I'm just fatassing around
Q: Well don't it's annoying.
A: I know. That's why I'm a fat-ass.