'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).