An unfortunate, ghastly incedent involving shit of some kind. Could be merely crapping one's pants. Up to and including unfavorable shit conditions of any kind.
"Holy crap, I just had the worst shincedent this morning! I had to EMERGENCY shit in a gas station bathroom so hard that it back-splashed up onto my shirt and I didn't notice. I actually proceded on to work and wondered what vile smell was following me around before my hot coworker informed me I was shit-stained!!!"