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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
 
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