Fit as a fiddle

This one is older than it looks, and "fit" originally didn't mean healthy at all. The phrase is recorded as early as 1616 as "as fine as a fiddle," using "fine" and later "fit" in their older sense of well-suited, well-arranged, or in good working order, the same sense that describes a well-tuned instrument. Over the following centuries the phrase narrowed specifically to physical health, its current meaning.