What does You Old Bat mean and where does the saying come from?

Here's a guess. It could be related to the Word "Batty" meaning crazed, insane or erratic. It comes from the term Bats in the Belfry because bats who lived in bell towers in Europe seem to fly erratically on occasions. "Old bat" is often used in the sentence "You crazy old bat." I suspect it was shortened to "Old Bat" because it is impolite to call someone crazy. It also seems to be reserved for women. I've heard people refer to an angry wife as the old bat as well as the old battle axe. In sum, I would suggest it is a less hurtful way of calling someone crazy either to their face or behind their back.