Butt Establishment Line

The line that defines the boundaries of your butt and your leg. It actually establishes where your butt begins (and where your leg ends). It can either be short or long, and some people don't even have one. People that have lower-set or saggy butts typically have this line. People that have higher-set or perky butts tend to not have this issue.

(Jack) Look at the butt establishment line on that girl! That's pretty disgusting!
(Jill) I know, her ass is so saggy! My butt is so perky that I don't even have a butt establishment line!