'Fit as a fiddle' means to be in very good health or physical shape. 'Fiddle' is the colloquial name for violin. There are different theories to the origin of this idiom which originates from the early 17th century. Some say that 'fit' was also used to mean 'suitable, seemly'. In the way we might say 'fit for a purpose' Others say that originally the phrase was 'fit as a fiddler'. (fiddlers were the street muscians who gave energetic performances and were therefore very fit ( in good health).

It means 100% fit. No problems at all with the fitness.

Thanks your answer

I think the reason is because the fiddle is shaped like a woman with a slim waist. 我觉得原因是因为小提琴的形状像瘦腰的女人的样子 。