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1.
The residue left on your finger when your hand strikes a turd as you reach back to wipe your heiney.
My hand hit the water when I reached back to wipe my ass and came up with knuckle mustard
by manofsteel249 January 25, 2011
 
2.
A fluid like substance originating from the penis of a overly tall half asian male.
Yo Chris you just got knuckle mustard all over Jonny's butt pussy.
by The Punanilope September 04, 2007