A girl with a huge ass. One's gauge is determined by taking both hands and extending both the thumbs and pinkies and putting one's thumbs together. The distance from one end of the pinky to the other is both one's gauge and the ideal width for a girl's ass. It is considered acceptable to go outside one's gauge within one hand length, but further than that is a complete "gauge buster" and should either not be done, or should be admonished by peers.
I got cowboy drunk last night and ended up busting the shit out of my gauge (with a gauge buster). I woke up praying she was gone in the morning, but she was in the kitchen shoveling cream cheese into her mouth with Ritz crackers.