nitty-gritty • n, The fundamentals, realities or basic facts of a situation or subject. The heart of the matter.
ORIGIN: according to the Oxford English Dictionary Online, the term started as US slang but its origin is unknown. Editor of the Dictionary of Slang, Jonathon Green, speculates it is merely a reduplication of the standard English word gritty.

ORIGIN 2: one theory is that "nitty-gritty" refers to the debris left in the bottom of a slave ships at the end of a voyage. Hence, use of the term is highly contentious and has been banned by the police.