late 14c., "crocket," later "small hook" (early 15c.), from Old French crochet (pronounced "crotchet") "hook" (see crochet). As a surgical instrument, from 1750. Figurative use in musical notation is from mid-15c., from the shape of the notes. Meaning "whimsical fancy" is from 1570s; perhaps from the same mechanical image in crank; but other authorities link this sense to the musical notation one.
