AssFault
n. AssFault is a more elegant and more sophisticated term to replace the relatively more vulgar ButtCrack or its synonym, AssCrack.

etymology: AssFault is a conjoining of two terms: "Ass," which refers to the posterior of a person, and "Fault" which is a geological term indicating a break or a crack.
After the long bike ride over the asphalt my AssFault was very sore... I think I need a new seat.

Dude 1: Dude-- pull up your pants. I can see your butt is broken!

Dude2: What?

Dude1: I can see your AssFault!