I’m thinking about adding this term to my vocabulary, since I’ve recently run into several people who can be described by the term. Anyone know?
Just a guess that “went bats” / “bats in the belfry” as a decades old metaphor for “crazy” morphed into “went bat + shit crazy”.
Off to GQ! (The mods there have bats in their belfries, so they’ll be able to relate.)
Cassell’s Dictionary of Slang, under “batshit,” gives to meanings: the one in the OP, and as a verb equivalent to “bullshit,” in the sense of gossip / tell lies. I’ve never heard “batshit” used as a verb, but then, I wasn’t around in the 60s.
Otherwise, they pretty well agree with everything else said in the two threads.