Explaining Idioms: 2) fit as a fiddle

The similie (明喻) 'fit as a fiddle' means 'perfectly fit', or 'in excellent condition'
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Fiddle is another word for violin (小提琴). I think you can say 'fit as a fiddle' is an idiom, as in a common phrase that doesn't seem to make sense, because we don't think of fiddles being fit.

The phrase relies on the notion of something being 'fit for use' or 'fit for purpose', but we mostly use it to describe people.

This expression dates from at least the 1600s. A well-tuned fiddle is in good shape and can play terrific music. If your musical instrument is not in good shape, then it won't sound right. In his 1616 play English-men for my Money, Haughton Williams writes: "this is excellent in faith, as fit as a fiddle".

Examples:

• Even at 80, Jacky was fit as a fiddle. 

•  She eats well and gets plenty of exercises. She's as fit as a fiddle.

In Popular Culture:

In the first half of the 20th century, a song called Fit as a Fiddle (and Ready for Love) was quite popular. It was written in 1932, before being recorded by several different artists, including Doris Day, over the next couple of decades. 

The song is probably most well-known for appearing in Singin' in the Rain as part of an elaborate dance scene with Gene Kelly and Donald O'Connor.