"Bat shit crazy" derivation

If you type this into google you get one useful link.

From everything2.com:

bat-shit crazy, a. (often abbreviated to bat-shit)

Of persons : Not of sound mind, mad, insane, deranged, lunatic. Also to describe any state of frenzied rage, violence, excitement or intoxication. No longer in formal medical use.

Of ideas : To describe any bizarre, unfounded, heret…