What is the X in X-mas?
"stirrup bone in the middle ear," 1660s, from Modern Latin (1560s), special use of Medieval Latin stapes "stirrup," probably an alteration of Late Latin stapia, related to stare "to stand" + pedem, accusative of pes "foot" (see foot). So called because the bone is shaped like a stirrup. This was an invented Medieval Latin word for "stirrup," for which there was no classical Latin word, as the ancients did not use stirrups.
stapes sta·pes (stā'pēz)
n. pl. stapes or sta·pe·des (stā'pĭ-dēz')
The smallest of the three auditory ossicles, whose base fits into the oval window and whose head is articulated with the lenticular process of the long limb of the incus. Also called stirrup.