Derived from a derogatory term for gay men when in the passive position during anal sex.
Now used to describe moments of intense ecstasy where pleasure touches pain.
Man 1:"Wow, that pill I dropped was fucking mental. If I had tits, I would have cum on 'em."
Man2:"Yeah, I know. You should have seen your face, you looked like a right pillowbiter."
In "The Adventures of Baron Munchausen" the king of the moon called the queen of the moon a pillowbiter when she was not laughing when he was tickling her feet.