Bat crap crazy

Where does the phrase "bat crap crazy" come from?

That's a slightly sanitized version of "bat **** crazy", which was discussed about a year ago. If youo type "bat ****" (with quotes) in the search box, you'll find the discussion.

Basically, the "bats" part comes from "bats in the belfry" or "batty", and "****" was added for emphasis.