fit as a fiddle - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]