Green’s Dictionary of Slang

1. (orig. US black) a white person, esp. a southerner or redneck.

1967		N. Mailer Why Are We in Vietnam? (1970) 13: You Talmud hokum, You clammy Have-it grit.
1976		P. Conroy Great Santini (1977) 390: ‘Good morning, grits,’ Bull roared as he entered the restaurant. [Ibid.] 461: Keep out of that nigger shit. You don’t want to get between the niggers and the grits when they go for each other’s throats.
1978		J. Webb Fields of Fire (1980) 136: ‘How’s the new Lieutenant?’ ‘A grit.’.

2. (US campus) a working-class white student.

1986		P Welsh Tales Out of School 98: A preppy wouldn’t be caught dead hanging around with a ‘grit.’ [HDAS].
1993		G.P. Pelecanos Nick’s Trip 17: [T]hat rare ability to fit in at the fringe of every group—hippies, grits, geeks, jocks.