Knuckle Mustard 

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

Knuckle Mustard by manofsteel249 January 25, 2011

knuckle mustard 

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.

knuckle mustard by The Punanilope September 14, 2007