This term sprang like a phoenix rising from the ashes from an incident that happened to a friend of mine. We'll call him Dude. Dude had the flu, and everything that comes with it, and was sprinting toward the bathroom for the millionth time with diarrhea when he realized he had his underwear on backwards. With no time to spare he proceeded to "do his business" through the fly opening or, to poop of out of the peehole.
As the phrase rolls off of the toungue so easily, it came to have many different applications.
*Parts of this story may have been dramatized for effect