What is the X in X-mas?
"jaw, jawbone," 1670s, from Latin maxilla "upper jaw," diminutive of mala "jaw, cheekbone." "Maxilla stands to mala as axilla, 'armpit,' stands to ala 'wing'" [Klein]. Related: Maxillar; maxilliform.
maxilla max·il·la (māk-sĭl'ə)
n. pl. max·il·las or max·il·lae (māk-sĭl'ē)
Either of a pair of irregularly shaped bones of the skull, fusing in the midline, supporting the upper teeth, and forming part of the eye sockets, hard palate, and nasal cavity; upper jaw.