Someone who is completely obsessed with some sort of cult fiction (Example: Lost, WoW, Star Trek, Napoleon Dynamite, Family Guy, Halo, Star Wars, Pokemon etc)
Girl: "Yeah, like he is Costelloed to it"
Guy: "That's pretty strong to say he's Costelloed"
Girl: "Well, he spends hours researching all the literature associated with it"
Guy: "You're right, he is a total Lost Costello"