Originally a British Royal Naval term. This was a long piece of rope, either frayed at the end, or with a bit of rag tied to the end of it, permanently fixed to a ship, which was dangled into the sea, and therefore, towed, next to the wooden plank with a hole in it which was secured over the edge of the ship, used as a toilet. After the sailor had finished his ablutions, he lifted the rope out of the water, and then wiped his rear-end clean with the wet, frayed end of the rope, and when he finished wiping, the soiled end of the rope was dropped back into the sea to clean itself as it was towed along by the ship. a Towed Rag.
Originally was Tow Rag from the red rag tied to the end of a truck carrying a long load (in England) or the rag tied to the rope towing another vehicle so people would not decapitate themselves by going through the gap.
Pretty well the rear end of everything and of no other use.
(Being poilte here)