Well who wouldn’t know their arse from their elbow, would you mind telling me that? For one thing an arse has two cheeks and is usually soft and an elbow is a sort of bony hard joint . Another difference is one is for attacking if someone comes up too close behind, the other well…. If someone gets too close to it from behind they might get the elbow in their stomach or somewhere below the stomach that might hurt alot!!.
But I do get the point. If one is totally knackered for some reason or other, say, they are working too hard or too many people are pulling out of them (like a mother sometimes for example) they might get driven to distraction and not know their arse from their elbow and end up giving someone an arse load of aggression and fondle them with their elbow!!!