Fit as a fiddle

In the late 16th century, from when the expression dates, fit meant fine or seemly and had nothing to do with physical fitness. A fiddle or a violin was seen as a fine musical instrument and the alliteration, no doubt, added to the popularity of the expression which meant as fine or as seemly as a violin and this meaning lasted until the mid-19th century. From this time onwards, the meaning changed and fit as a fiddle now refers purely to physical fitness, which is most odd when one thinks about it, because how on earth can fiddles or violins be symbols or metaphors for physical fitness?