A subspecies of turtlehead that occurs when the sphincter has been successfully employed for a time in confining a turd to the rectum. As control is gradually lost, a tiny aperture appears at first, allowing for only a very thin extrusion of feces. As the butt-muscle fails in the continued absence of a toilet, the aperture quickly grows in circumference over the next inch or two of escaping dookie, creating a rough cone-shape, resembling the head of a rat more than a turtle.
Jim: "Next rest area 67 miles dude, how's yer ass holdin' out?"
John: "I was doin' good for a while man, but now I got a serious rathead pokin'..."