Simply put, explosive diarrhea. The term is usually applied in the instance where the perpetrator doesn't "make it" into the fully seated position prior to detonation. The result being a completely fouled facility - floor, walls, possibly the ceiling, etc.
The haz-mat team was called in to clean up the men's room after someone shatter blasted the stall.
If someone are saying/said something stupid/weird, you can say «shatcha blest» to make them silent, or make them cautious about the stupid/weird thing they are saying or said.
You can also use the saying against someone if they are doing/did something stupid/annoying/weird.
I told Johannes «shatcha blest» because he said something stupid that annoyed me.
I told Johannes «shatcha blest» because he did something stupid, or because he acted different in a weird way.