Green’s Dictionary of Slang

(US) the essence, the basics, the ‘brass tacks’; thus meat-and-potato adj., basic, essential.

1960		J. Havoc Early Havoc 56: The place teemed with excitement. This was meat and potatoes. A nice lively cockfight.
1967		Maclean’s (Toronto) July 65: Textbooks remain the meat and potatoes of publishing in Canada.
1975		L. Rosten Dear ‘Herm’ 17: I mean real meat-and-potatoe stuff.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 122: Tryone said lots of things, but let’s get to meat and potatoes.