A girl that spends exorbiant amounts of time at a frat house, usually with the intent to slut it up with one or more of the fraternity brothers.

A girl that fucks a different frat guy every week.
1. Michelle S. at Illinois State University

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."
by Dave December 13, 2004
A licentious girl who tends to spend copious amounts of time at a fraternity looking for ”frat guys” to pound her in ways that are illegal in most conservative states.
1. Kristen M. at Purdue University

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.
by flouster April 23, 2009
a girl who spends so much time at a particular frat that she is practically a member. a girl of bro status who has been accepted by the brothers of a fraternity as one of their own.
don't mind her, she's just here broing out, being her standard frat rat self.
by #miamifratrat March 30, 2011
1. A greek system leech. Someone male (could be female) who is not an actual member of a particular Frat, but comes to every Fratty Function anyways. Most often these people wouldn't be accepted by the frat anyways and no one really wants them around.
FrattyFrat #1: So you think John's showing up tonight?
FrattyFrat #2: Dude, when doesn't that Frat Rat show up?
FrattyFrat #1: Shit... oh well, lets go rape some high school girls.
by Rob March 28, 2005
a girl that whores herself at a frat any day of the week...for free.
a frat rat is a slut, paris hilton, briana hilton who will suck off pretty much anyone
Any member of a college social fraternity. Usually derogatory.
#1: I heard John joined SigApes.
#2: Yeah, he's become a frat rat.
by Doug Hall October 3, 2006
Some one who is known for sleeping with many guys in the same fraternity.
"Hey Frat Rat, can i be next?"
"what the hell are you talking about?"
"I've heard you've slept with every guy in this Frat."
by Annie-mouse November 10, 2012