A loaf of shit that is beginning to poke out of your ass prior to reaching a toilet. Turtleheads are generally dealt with using careful restriction of the sphincter muscles until you can make it to a toilet. You must squeeze firmly enough to prevent the shit loaf from further sliding out of the ass, but not so firmly that you chop the turtlehead off, or smash it with your ass cheeks creating an undesirable mess.
Guy 1: "I really need to take a shit."
Guy 2: "A little, or a lot?"
Guy 1: "Well, I've got a turtlehead."
Guy 2: "Okay, that's an emergency. Be careful not to pinch it off. I think I see a restroom over there."