nitty-grittynitty-gritty

The story

Of uncertain origin; said to have been first used by black jazz musicians from the United States, the word is perhaps a reduplication of gritty (‘resembling grit’) (ultimately from Proto-Germanic *greutą (‘grit’), from Proto-Indo-European *gʰrewd-) with alteration of the first syllable.1uncertain The source flags this step as uncertain. An urban legend links the origin of the word to the debris remaining in the holds of slave ships after the slaves had been disembarked, but there is no evidence for this.2folk etymology The source marks this as unproven or a folk etymology. Quoted, not asserted. The word is not attested early enough to have been associated with slavery.

Full etymology on Wiktionary