Green’s Dictionary of Slang

1. lies, nonsense, rubbish; also as excl. [play on bullshit n. (1)/crap n.¹ (4)].

1943		H. McDiarmid Lucky Poet 100: Batshit ! ' That is exactly what the judgement of almost every writer of signed reviews in Great Britain amounts to. Just that !
1946		N. Gidding End Over End 34: Forty miles from Rome; sixty-four kilometers from [...] those round-bottomed, big-breasted Italian girls, anxious to give their all for the evading American airman. Batshit.
1946		N. Gidding End Over End 161: Batshit. You’ll mever get a cigarette that way .
1965		H. Rhodes Chosen Few (1966) 140: Bill says [...] ‘Maybe a good licking will help keep ’em in their place.’ ... BATSHIT!!!
1971		R.D. Pharr S.R.O. (1998) 173: ‘Batshit,’ I said and [...] I meant every syllable.
1979		(con. 1949) J. Hurling Boomers 61: Batshit! Now whut the hail’s goin’ on out there with them kids!
1991		W.T. Vollmann Whores for Gloria 124: Was I talking bullshit or batshit or dogshit or was it just ratshit?
2003		Guardian 10 Dec. 40/2: They just don’t care and have bat shit for brains.

2. an insane person [bats adj. + apeshit adj. (1)].

1982

(con. 1970) J.M. Del Vecchio 13th Valley (1983) 309: ‘“Fuck you” is two words, bat shit.’ [Ibid.] 310: ‘You ain’t in Saigon, bat shit, you up the Sông Bo.’.