As fit as a fiddle is a long standing English idiom meaning very healthy or in excellent condition. Today, many people hear fit and think of exercise or athletic ability, but the older meaning of fit was broader. It could mean suitable, proper, or in good working order.
That older sense helps explain the phrase. A fiddle, or violin, sounds best when it is properly tuned and ready to play. So the comparison suggests someone who is well formed, in good condition, and functioning as they should.
The exact origin is not fully certain, but this explanation fits both the history of the word fit and the image behind the idiom. It is less about muscles and more about being sound and well.
- Example: After a week of rest, he felt as fit as a fiddle.
- Contrast: In modern speech, fit often means physically athletic, but in this idiom it carries an older meaning.
That is why the phrase can describe general health, even when no exercise is being discussed.