A small part attended to on its own. Detail is where a general claim either holds or fails.
French détail (a cutting into pieces), from détailler (to cut up), from de- (apart) + tailler (to cut), from Late Latin taliare (to cut); a small part attended to on its own, cut apart from the whole to be examined closely.
French tailler is to cut, and a detail is a piece severed from the whole precisely so it can be examined alone. A claim too coarse to survive being cut this way was never as solid as its general shape suggested.