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
"Her pussy was three knuckles deep--pretty impressive."
Usually just the tip of the finger is used; but is someone is really going after the gold; they will be 'knuckles deep'.