What does "fit as a fiddle" mean — and why is it funny?

informal

Meaning

In very good health and excellent physical condition.

Where it comes from

From the 1600s. 'Fit' here originally meant suitable or well-suited, and a fiddle was praised as a finely made, well-tuned instrument.

Why it is funny

To modern ears the comparison is charmingly nonsensical — a violin has no health to speak of. The phrase survives mostly on its bouncy alliteration, and we say it happily without ever asking why a fiddle should be the picture of fitness.

Used in a sentence

"My grandmother is ninety and still fit as a fiddle."