Knuckling

[Britney]

insertion of ones fingers in anothers anus past the middle knuckle

[Mark Zuckerberg]

The act of [stimulating] the clitoris by vigorously rubbing ones [knuckles] back and forth in a [sawing] motion.

[Oralee]

Knuckling is the act of 2 bros [fist bumping] eachother. As the two bros [fist bump] eachother, a third bro proceeds to slide his dick between the other bros touching [knuckles]. He then proceeds to fuck their knuckles.

[Becky]

That act of pleasuring a woman by making a fist and using ones [knuckles in] a [sawing] motion to stimulate [the clitoris].