1
general::
In excellent form or health. For example, He's not just recovered, he's fit as a fiddle. The original allusion of this simile has been lost. Its survival is probably due to the pleasant sound of its alliteration. [Early 1600s]
American Heritage Idioms
2
general::
Phrase(s): *fit as a fiddle
Cliché in very good health. (*Also: as ~.) • You may feel sick now, but after a few days of rest and plenty of liquids, you’ll be fit as a fiddle. • Grandson: Are you sure you’ll be able to climb all these stairs? Grandmother: Of course! I feel as fit as a fiddle today.
McGrawhill's American Idioms And Phrasal Verbs