Green’s Dictionary of Slang

(US) the basic facts, the core.

1964		K. Kesey Sometimes a Great Notion 182: When it comes right down to the nuts of it.
1969		M. Puzo Godfather 133: ‘You’re right, kid,’ he said. ‘You got right to the old nuts.’.
1970		N.Y. Times in Barnhart Dict. New Eng. (1973) 325/2: Just the way it’s always been, but without tackling the nut of the problem.
1989		Joan Rivers Show [synd. TV series] The nut of the show is just that you have to deal with your problems [HDAS].
1991		CNN News 5 Dec. [CNN-TV] Attorney Black did not get to the nut of her testimony [HDAS].