What is Fat Rat?
1.
A Guinea Pig... A vile little creature that resembles nothing so much as a rat, if that rat were designed by GM and built by AMC.
Mom: So... Would you like to get a pet?
Kid: Sure. Anything but a fat rat.
A: What -is- that smell?
B: Oh, that's my brother's fat-rats. He changes their litter about as often as he showers.
See
2.
An overweight woman.
Geez, are all Tri Delts fat rats?