Term used to describe the last turd in a pooping session. More specifically, the turd that you didn't know was coming until you stand up and begin to fasten your pants, much like the slowest cow in a moving herd.
Joe: Dude, are you done in the bathroom? We're gonna be late!
Tom: Yeah, I'm comin'! I thought I was done, but I had a behind-the-herd turd.