The act of cramming your finger (at least to the 3rd knuckle) inside of someones 4th eye. If done correctly and your partners 4th eye is left quivering, your finger should look like an Alabama Mudknuckle!
A mudknuckle is when you take a shit, smear your knuckles in it an then punch someone in the face.
Mudknuckles can be also delivered using other people's shit, dog shit, cat shit and/or tiger shit. Basically you can use any poop available.
VARIATIONS:
A Hairy Mudknuckle is when you dip your shit covered knuckles in pubic shavings before punching someone in the face.
A Mudknuckle Favor is when you smear your knuckles in shit and then pee on your knuckles to rinse some of the poop of before punching someone in the face.