Joey Malsky raised an interesting question in regard to the phrase be in fine fettle.
Fettle: It occurs to me that the otherwise nonsensical phrase fit as a fiddle could have been derived from in fine fettle, preserving the sense while using a more familiar word. (What’s that called again?) Is there any evidence of this transition?”
Replacing an unfamiliar borrowed word with a more familiar one is called folk etymology (French crevisse becomes crayfish in English). But we have no evidence of folk etymology being involved in the rise of fit as a fiddle.
I have three books that discuss this idiom and all say the same thing: no one knows why fitness and fiddles are associated but the association goes back to the 17th century. The Oxford Dictionary’s earliest citation is 1603 (fit as a farthing fiddle) but no explanation of why fiddle rather than mud duck or saxaphone. In the dozen or more examples OED gives, none confuse fiddle with fettle. If one or two had confused them—if there were examples of in fine fiddle for in fine fettle, we would have a basis to suspect that they are related.
This expression fit as a fiddle is one of an large lexicon of crystalized (which is to say, idiomatized) manner adverb phrases:
- crazy as loon
- sly as a fox
- hungry as a wolf
- quick as a wink
- greedy as a hog
- strong as a horse
- skinny as a rake
- sick as a dog
- crooked as a snake
- straight as an arrow
- clean as a whistle
- quiet as a mouse
- wise as an owl
—just to mention a few off the top of my head. As you can see, there isn’t much of a pattern here, just folk prejudices, so there is no reason why we would expect one for fit as a fiddle.
Of course, we love alliteration and first syllable of fiddle being almost identical to fit gives fiddle a slight edge on being chosen as the simile for fit. However, etymology has nothing beyond that to say for the relationship.