The phrase “fit as a fiddle” means: in fine form or condition; in splendid health.
Although “fiddles” were known in England back at least in the early thirteenth century, it was some four hundred years later, evidently, before their shape, form, tone, and other qualities became so pleasing as to invite complimentary applications to humans.
“To have one’s face made of a fiddle” was to be exceptionally good looking.
“To play first fiddle” was to occupy a leading position, and one “fit as a fiddle” or “fine as a fiddle” was beyond further need of improvement in health or condition.
Apperson traces the metaphor to Haughton’s England for My Money (1616), “This is excellent, i’ faith; as fit as a fiddle.”