A guy who enjoys anal sex. It refers to the guy on the giving end, not the man or woman on the recieving end. This term is used to discribe gay and straight men. A well known synonym is "fudge-packer".
Guy 1: "Dude, i was with this chick the other night. Long story short, I licked her asshole to get her excited and then I stuck it in her ass to finish her off."
Guy 2: "Wow, I had no idea you were a Keebler Elf."