Usually said in a really bad italian accent, is used to discribe the state of ones mind after a massive drug binge especially after back to back nights of no sleep.
To eject fecal matter with such speed and ferocity that it rebounds on whatever surface you are aiming at and hits you in the behind, quite like a boomerang.
I just had the worst scattarang ever. It bounced right off of Phil and I got bruises all over my frikken balls.
Dude, don't eat at that shitty place, you'll scattarang for sure.