Butt Girl

A girl at the gym who only does leg day and doesn't work out any other parts of her body. As a result of her routine, she has become a butt walking on two feet.

Damn, you see that girl over there leg pressing 300 pounds in the dainty spandex outfit that's as slim as a toothpick and an ass big enough to set a glass of water on over there? That right there is a butt girl.