Code for someone to admit that they are sexually attracted to men. Often used when someone is gay, but too scared to come out. They will often deny it if you ask them whether they are gay or not.
Originated by observing the way bats defecate. Bats hang upside-down and poo upside-down. If people were to hang upside-down and poo it would go all over their face therefore only a crazy person would shit like a bat making them batshit crazy.