Green’s Dictionary of Slang

(US campus) an excl. of agreement or of sarcastic dismissal.

1967		R. Conot Rivers of Blood 93: ‘That be real groovy!’ ‘Big shit!’.
1979		H. Feldman et al. Angel Dust 167: Being burnt was, yeah, he’s burnt, big shit. It’s the most stupidest thing, being burnt.
1989		P. Munro Sl. U. 36: After Julie told Tara her big news, Tara replied, ‘Big shit!’.
2001		Sydney Morning Herald (Aus.) 6 Jan. n.p.: So here’s a tentative guide to Sydney teenspeak: [...] Big S--- Sherlock (who cares, leave me alone) .