“Nitty gritty” is often claimed to have its source in slavery, as a derogatory reference to the English slave trade of the 18th century. The phrase is usually used with the prefix “getting down to the…” and there is a sense that, whatever the “nitty gritty” is, it is at the bottom of something.

The suggestion is that it originated as a term for the unimportant debris left at the bottom of ships after the slaves had been removed and that the meaning was extended to include the slaves themselves.

There is no evidence to support the suggestion that “nitty gritty” has any connection with slave ships. It could equally have originated in the USA as an African-American expression.

“nitty gritty” isn't even recorded in print until the 1950s, long after slave ships had disappeared, and none of the early references make any link to slavery.