Green’s Dictionary of Slang

(US) an excl. of disgust, disappointment, disbelief or denial.

1926		E. Hemingway letter 2 Jan. in Baker Sel. Letters (1981) 186: I have a low middle class upbringing that makes me feel uncomfortable addressing such remarks as horse shit to a lady.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 210: ‘Horse,’ said Paulie.
c.1935		‘The Rubber Salesman’ [comic strip] in B. Adelman Tijuana Bibles (1997) 50: ‘If you had followed my advise [sic] [...] you would have been OK.’ ‘Ah, horse shit!’.
1944		R.L. Bellem ‘Killer’s Cure’ Hollywood Detective Mar. 🌐 I favored them both with a scoffing sneer. ‘Horse chestnuts’.
1958		T. Berger Crazy in Berlin 195: He stifled the impulse to say ‘Horseshit!’.
1969		G. Shelton ‘Fabulous Furry Freak Bros.’ [comic strip] ‘This is definitely the weekend of the big bust.’ ‘Horsehit!’.
1974		(con. 1940s) E. Thompson Tattoo (1977) 216: Horseshit! Christian’s ass!
1988		(con. 1940s) C. Bram Hold Tight (1990) 187: ‘I’ve always been Lena.’ ‘Horseshit! You’re Juke in a dress.’.
2005		D.H. Wilson Pseudo-City 58: Mr. Krapps pretended he had not known the two men were there. He turned around with a surprised look on his face. ‘Oh! Hello gentlemen. I didn’t realize you were here.’ ‘Horseshit, Mr. Krapps,’ griped Mr. Schmotzer.
2009		T. Black Gutted 42: ‘Mac, I can’t do that.’ ‘Horseshit! I’d scarper’.
2014		M. McBride Swollen Red Sun 103: ‘I don’t know how to repay you.’ ‘Aw, horseshit, Olen [...] you don’t owe me a dang thing’.