Something that is used to wipe the ass when conventional toilet paper is not available. Most often, the need for TP Backup is when the person pooping misjudges the amout of toilet available at the time. Hand towels, magazine pages and discarded tissue are often useful TP Backup.
When the toilet paper ran out, I realized that there was still more poop to clean off my ass. I looked around the bathroom and saw a Cabbage Patch Doll, I had to use it as TP Backup , or else i'd be in big trouble later.