A girl that fucks a different frat guy every week.
2. Frat guy 1: "Man that chick is over here doing a different brother every week!"
Frat guy 2: "Yeah dude. She's a frat rat."
2. Frat Meathead 1: Yo Brah, that Sarah has been shaking around here brah.
Frat Meathead 2: Yeah man, that girls a frat rat … brah.
FrattyFrat #2: Dude, when doesn't that Frat Rat show up?
FrattyFrat #1: Shit... oh well, lets go rape some high school girls.