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1.
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!
by Neologist-PJG November 15, 2012
 
2.
A disgusting feature that makes a relatively nice ass look bad. (hair, mole, boil, hemmeroid, etc.).
Cellulite is a major assfault. It can make the nicest asses go bad.
by bLiTch June 19, 2008
 
3.
a violation of etiquette that occurs when the posterior of the person seated next to you (typically on an airplane flight) crosses over into your assigned seat, impinging upon your territory, privacy, and comfort.
"The obese man got the last seat on the plane next to me, and he committed an ass fault when his butt spilled over into my seat."
by FactCheckGirl April 07, 2010
 
4.
1. like a foot fault in bowling this kind of fault is because of your ass and this isn't crossing the line of where you cant step this is crossing the line of how the air should smell
an ass fault is a fart
by Kleets McCool July 30, 2011