early 14c., "small hook;" mid-15c. "a staff with a hook at the end," from Old French crochet (pronounced "crotchet") "small hook; canine tooth" (12c.), diminutive of croc "hook," from Old Norse krokr "hook," which is of obscure origin but perhaps related to the widespread group of Germanic kr- words meaning "bent, hooked."
As a curved surgical instrument with a sharp hook, from 1750. Figurative use in musical notation for "quarter note" is from mid-15c., from the shape of the notes. Also from 1670s in now-obsolete sense "one of the pair of marks now called 'brackets.'"
Meaning "whimsical fancy, singular opinion," especially one held by someone who has no competency to form a sound one, is from 1570s; the sense is uncertain, perhaps it is the same mechanical image in extended senses of crank; but other authorities link it to the musical notation one (think: "too many notes").