the back rim of the toilet underneath the toilet seat. Often shit splashes back there and sticks, especially during diarrhea dumps. Many people will not even bother to wipe the shitsplash off after blasting out a diarrhea shit - to the disgust and dismay of the next person that needs to use it. This word is derived from the term "backsplash" - the wall in the kitchen between the counter and the bottom of the cabinets where prepared food will often splash against during preparation. However - in this case - its shit splashing against it.
Joe took a dump in the public bathroom in the mall - it was a huge diarrhea explosion - when his ass exploded shit splattered all over the back wall of the toilet- also known as the shitsplash after he dumped, he flushed the toilet and shit residue was still there. rather than wipe it off he left it there to the dismay of the next person that used the toilet.