When a heavy and/or furiously excreted turd hits the toilet water so hard that it causes a splash of water to jump up and stain your upper body clothing.
God damn Son! I suffered a major back splash when i dropped the kids at the pool just now! All my shit be shitty as a shitty assmutha fucker!
A back splash is when you're taking a dump, and the crap makes a splash and hits you in the anus.
Usually very cold water unless you've peed in it and warmed it up.