What is the origin of the idiom "fit as a fiddle" and when to use it?

The idiom "fit as a fiddle" is believed to have originated in the 17th century. It is used to describe someone who is in excellent physical health and condition, often indicating a high level of fitness and well-being, just like a well-tuned musical instrument (a fiddle or violin) that produces the best sound when it is in perfect condition. This phrase is commonly used in a positive and complimentary manner.