What does it mean for someone to have "a butt that won't quit"?

Just noticed I was the first one to bring up the Simpsons gag upthread.

Relevant to this, I guess, but one of the first things I saw on FB today was an Instagram share from a actress/dancer friend of mine who's having to take pole dancing lessons for a regional production of Rock of Ages she just got cast in. In said photo, she's doing a handstand against the pole and...well, yeah--BUTT THAT WON'T QUIT.

always seems to have a dick in it was robbed