Green’s Dictionary of Slang

(orig. US black) fundamental, basic.

1964		D. Gregory Nigger (1965) 77: While I watched, the boys on the football team, who had played against those hoods, been down in the nitty gritty dirt with them, stepped out with their dates.
1971		B. Moyers Listening to America 93: People who talk about brotherhood from their high-rise apartments expect this guy living in the nitty-gritty deteriorating neighborhood to take the rap for them.
1980		J. McClure Spike Island (1981) 338: I’m gettin’ in on the real nitty-gritty part of it!
1994		Rebennack & Rummel Under A Hoodoo Moon 193: It [...] meant a whole lot more work on those nitty-gritty details.
2000		P. Beatty Tuff 86: What makes you think you’ll be able to mingle with the nitty-gritty element?