Fit as a fiddle means in excellent health. Right as rain and fit as a fiddle, still fit as a fiddle, fit as a fiddle at eighty.
It is said most often of the old and the recovered, which is where the pleasure of it lies. The origin is unknown and the alliteration is probably the whole explanation, since fit once meant suitable rather than healthy.
1phrasefit as a fiddleIn excellent health.
“After his knee operation, Grandad is fit as a fiddle and walking every day.”
fit as a fiddle is a health.
Of unknown origin, probably alliterative; fit once meant suitable rather than healthy.
unknown; possibly alliterative.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms