look up any word, like eiffel tower:
 
1.
when a person shits on another persons face and then punches that person in the face...resulting in muddy knuckles

also could be after fisting an ass and resulting in muddy knuckles
bitch you keep sass'n me were gonna play muddy knuckles
by bukake_ninja June 24, 2008
 
2.
Anal fisting: where some one inserts a clenched fist into someones anus.
Clarissa's bum hurt after her boyfriend gave her a muddy knuckle.
by Rhyzzzzzzzzzzzzzzzzzz December 01, 2009