What is Once Fat Bitch?
a girl (or guy) that used to be really fat, but finally grew into their body and now looks pretty decent. They usually change from a nice fat kid to an arrogant/cocky prick.
Person A: Back in the day that girl used to eat cookies with me in the cafteria.
Person B: Yeah, now she ignores you and eats salad for lunch. She's a once fat bitch.