Green’s Dictionary of Slang

a general term of dismissal, disdain.

1904		Marvel 22 Oct. 7: ‘Eat it!’ cried Pete, giving him some cuts that made him howl.
1953		L. Uris Battle Cry (1964) 315: ‘Eat it,’ Levin spat.
1974		G.V. Higgins Cogan’s Trade (1975) 14: In red spray paint were irregular letters [...] ‘southie eats it’.
1977		A. Hoffman Property Of (1978) 204: ‘Eat it,’ Tony said quietly.
1980		W. Kotzwinkle Jack in the Box 115: He’d only learned how to signal Fuck You and Eat It, Mac.
1982		(con. 1970) J.M. Del Vecchio 13th Valley (1983) 579: ‘Eat it, Lieutenant [...] We ’re coming up now.’.
1983		R. Price Breaks 51: I whispered into the mouthpiece, ‘Eat it, you slut.’.