A rim of fat that bulges out above a "pleasantly plump" girl's too-tight pants. These girls usually don't realize that their outfits are completely unflattering, so it's best not to point this out. In today's modern McDonald's fed society, muffintops are not a rare sight.
As Tracy walked down the street, the fatty muffintop peeking out from below her wife-beater jiggled wildly. It was rather entertaining.