A girl with bad fat eats fast food every evening followed by a Ben&Jerry's. This contributes to bad fat which gives her a huge flat ass, hanging boobs, sticking out belly, and in the more severe cases a double chin.
These types of girls are bad catches and should be avoided, unless she is a friend of a hot girl of course...
T: Hey man, check out that girl's ass! I wish i could rip those jeans off and do her all night!
J: Nah man, that's bad fat... You don't want a sumo wrestler doing you all night do you?