A 'skullfucker' is someone who fucks you in the skull. Either in the eye socket, or mouth, or anywhere else where the penis can penetrate the skull, such as a hole smashed in the back of the skull with a hammer so you can feel the brains on your cock and shoot semen directly onto the area that gives people horrifying nightmares of dicks entering their skulls and shooting semen into their brains to impregnate them with the children of the master race of skullfuckers.
Suzy's a slut, she'll give you skull for weed.
Ashley skulled me all night till I busted in her throat.
Ethel has a glass eye so I skullfucked her socket and lubed it up with my semen for her glass eye. HAHAHAHAHAHA!