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.
- (vulgar, literally) fecal matter produced by bats
- (slang, vulgar) nonsense, horseshit; especially when said by or attributed to someone irrational.
- (slang, vulgar) extreme irrationality or unreasonableness, insanity, craziness.
- (slang, vulgar) extremely irrational or unreasonable, insane, crazy.
- Don't take any courses from that professor. She's completely batshit.
batshit (not comparable)
- (slang) Used as an intensifier, particularly for insane or synonyms.
- When we heard about it he went batshit nuts.