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A form of measurement used to determine the size or width of a vagina or anus, or the act of inserting fingers into said orifices. Preceded by a number which indicates how many knuckles.

---It still has not been determined if fisting someone is considered 4 or 5 knuckles deep, as the debate contiunes if the thumb counts as a knuckle
"So last night I was two knuckles deep in this chick's ass..."

"Her pussy was three knuckles deep--pretty impressive."
by Kevin Kugler March 01, 2006
A term to describe how much of a finger (or fingers) length is inserted into a persons nose; whilst they are picking a booger out of it.
Usually just the tip of the finger is used; but is someone is really going after the gold; they will be 'knuckles deep'.
"Damn, that bum was knuckles deep in his nose"
by Tyler Turden3 October 17, 2008