Knuckle Sludge

Extending one's finger and ramming said digit into the anus until the knuckle reaches the rectum.

Nothing beats reruns of "Welcome Back Kotter" and gettin' the ole' Knuckle Sludge.

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sludge knuckles

Shit residue, when you wipe but it's too slippery and you slide your knuckles in your dirty butt crack leaving a smear upon the the knuckle.

"Bro I just tried to wipe my ass, now I got sludge knuckles."