Where does the term "bat sh!t crazy" come from? I obviously know what it means, and I sometimes use it in everyday conversation, but why bat poo? What's so crazy about that? Because they hang upside down? Maybe, I'm trying to be too logical about this, but I've just been wondering. I searched urbandictionary, but they weren't any help. Once I get something like this in my head I just can't forget about it until I know the answer. TIA!