When someone is unattractive but almost everyone finds them unbelievably sexy for no apparent reason. Though the "sexiness" may be linked to their voice, actions, charisma, and so on.
Girl1: Oh my god, he is so sexy!
Girl2: What, you think he's good looking?
Girl1: No, he's hideous.
Girl2: Then how do you find him sexy?!
Girl1: I don't know...It must be the 'Graverobber Effect".
Girl2: Oh, okay.