Everybody has a shitty day every once in a while. Sometimes when nature calls one can find that adequate facilities are too far away to be reached in time. In a pinch, you can rip off your shirt pocket and use it to wipe your ass when no toilet paper is available. A SHIRT SHART is the process of crapping and the using your own shirt pocket to wipe up.
Yeah, I got caught up in a shirt shart yesterday. We were fishing at the lake and I had to take a dump. I gotta go to Old Navy now to replenish my wardrobe.