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1.
The growth of an inflammed, festered, and bloody hemorrhoid that creates a groove-like affect in your feces.
Dave, don't do it. Jeff has an ass knuckle, that's not going to be fun.
by Sean Piner September 30, 2006
 
2.
Someone who is clueless or stupid.
"Dude the new guy is such a dumbass!

"for real, he's a total friggin' ass-knuckle!
by Skin bone May 21, 2011