Green’s Dictionary of Slang

(US) a psychotic.

1970		in P.R. Runkel Law Unto Themselves 93: These apeshits that go aroun’ lickin the cunt of little girls.
2001		‘Randy Everhard’ Tattoo of a Naked Lady 83: Trust is overrated, apeshit. [Ibid.] 195: That apeshit wasn’t laughing anymore.