Clavicle, the collarbone: the slender, sigmoid-shaped bone that links the manubrium of the sternum to the acromion of the scapula. From the Latin clavicula, ‘small key’, a translation of the Greek kleidion.
The word was coined in the 12 century by the translators of the 10 century Persian physician Abu Ali Sina (his name was often latinized as Avicenna) whose works were a mainstay of European medicine the until the mid-17th century. Ali Sina used alchiab, ("the key") for collarbone, this from his own translations of Latin anatomy treatises from ancient Rome.
Ala Sina's translators returned to the Latin, renaming alchiab the clavicula, the diminutive of clavis (key).
But why key? A common tale is that the Romans likened the collarbone to a clavis simply because of similarity in appearance. However, it was probably not Roman door keys, which do not resemble clavicles, that were alluded to but rather S-shaped lift-latches, a primitive type key type first seen the Iron Age and common in Rome.
designed to allow a door-latch
to be lifted through a hole in the door.
It acted as a simple key. England, ca 350 BC
It has also been suggested that the relationship of the clavicle to a key is metaphorical, given that the bone "locks" the shoulder girdle to the trunk. Of course neither of these explanations are mutually exclusive, as the key shape lends itself nicely to the metaphor.
Perhaps though, "key" was a mistranslation by Ali Sina: yet another explanation is that the name comes from the resemblance of the bone to a toy, a curved stick called a clavicula or clavis trochi used by Greek and Roman children to roll, or trundle, a hoop.
A clavicula or clavis trochi, a child's toy for trundling a wheel in ancient Greece and Rome; from the Dictionary of Greek and Roman Antiquities, 3rd Edition by Anthony Rich. 1874
A minority view holds that the term has nothing to do with keys or trundles or metaphors but rather is named for the clavicle's resemblance to the sinuous tendrils of some plants. The Latin word for tendril is also clavicula.
The original Greek kleidion lives on in sternocleidomastoid muscle
Anatomically unrelated: the clavichord was the first keyboard instrument.