When a girl has large dimple like indentations on her lower back directly above her butt. This results in a sometimes sexy, strange occurance called the back butt.
"Damn, I don't know wether to be turned on or throw up at the sight of Lexxy's back butt."
"I hate summer. I don't like to show off my back butt. Seriously, aren't dimples supposed to be on your face?"