Usually pertaining to butch lesbians, a knucklebuster is someone who forcefully, and rapidly inserts their fingers (usually 2-3) into a woman's vagina, and in doing so they injure the joints of said fingers. Medically speaking, it's usually a small strain in a joint, not a knuckle, but the nomenclature holds steadfast.
A derogatory reference to a lesbian, or a girl who won't put out right away
The direct result of hardcore "fingerbanging"
Bummed Dude: No, she was a goddamned knucklebuster