(originally US, colloquial, also attributively) The core or essence of something; the gist.
He gave a short summary without getting into the nitty-gritty of the problem.
adjective
(colloquial) Dealing with something in great depth.
1976, United States. National Commission on Arthritis and Related Musculoskeletal Diseases, Arthritis - Out of the Maze
We deal with them at a very, very nitty gritty level, and we deal with the results of years and years of problems that they've had in the past.