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2.
Pertaining to a situation when you are hot and heavy with a significant other and your fingers are "Knuckledeep" in their anal or vaginal opening
She was laid out on the bed and i was knuckledeep in her vagina when her sister walked in.
by Charles Penguin March 04, 2008
 
1.
The act of using fingers to pleasure a female vagina and/or clit.
I drive my girlfriend crazy by shoving my tounge down her throat and going knuckle deep.
by Andy Holman October 09, 2005