from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. fecal matter produced by bats
- adj. Too irrational to be dealt with sanely.
- adv. Used as an intensifier, particularly for insane or synonyms.
Presumably from batty ("crazy"), itself from earlier have bats in one's belfry, from tendency of bats to fly around erratically. Possibly influenced by or derived from apeshit, particularly in phrase go apeshit. (Wiktionary)