Hillbilly Circumcision

Alright, listen up, buttercup. This "joke" is about hillbillies, who are often stereotyped as, well, not the brightest bulbs in the box. The "circumcision" part is a distraction. The punchline is just a violent act against his sister, playing on the stereotype of hillbillies as unsophisticated, violent and also potentially as incestuous. And based on the fact that you are reading this explanation, it is safe to assume that you are a moron.