Originally from the Horribleville comic by KC Green, Dickbutt is the name of a cartoonishly drawn penis, complete with testicles, that has another smaller penis and testicles protruding from it's backside. The phallus in question has become a sort of meme thanks to it's simplistic "derp" face, and because of the sheer absurdity of the concept.
Since it is a representation of a penis, it should probably be tagged explicit in all cases where it can be clearly seen.