fiddle可以指类似小提琴的乐器。Fit可以指健康的，健壮的。Fit as a fiddle的意思就是身体很好、非常健康、充满活力。

【例句】
He is as fit as a fiddle though he is already seventy.
尽管已经七十岁了，他还是非常健康。

I did have a surgery a few months ago, but I'm as fit as a fiddle now. 
我确实在几个月前动了手术，但现在我身体很好。