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1.
After someone has fisted another person, they leave their hand uncleaned for a while. The fluids, dry and harden on the hand, forming a Crusty Knuckle, or Crusty Knuckles depending on the severity.
Jasmin fisted Leeroy. She left her hand overnight an the shit hardened. In the morning her hand had turned into a Crusty Knuckle.
by L.Jenkins January 10, 2012
 
2.
A crusty knuckle is when you have your girl stimulate your prostate by shoving her middle finger up your anus. Look at her knuckle once she is done.
I blew a load so hard while Christina was giving me a Crusty Knuckle.
by John Chapman May 16, 2007
 
3.
A condition which sets in after a prolonged session of personal masturbation and falling asleep afterwards. The knuckles of the hand are glued into position from the secreted bodily fluid and is nearly impossible to reverse.
As Jim lay in his favorite recliner, he was awakened by his mothers entrance into his room and was caught with crusty knuckles.
by Mike Bruce October 03, 2005
 
4.
pedophile.
man that guy is a fucking crusty knuckles man.
by REVERENDJOY September 17, 2008