What is the X in X-mas?
1706, from Latin alveolus "a tray, trough, basin; bed of a small river," diminutive of alvus "belly, stomach, paunch, bowels; hold of a ship," from PIE *aulo- "hole, cavity" (cf. Greek aulos "tube, pipe," Old Church Slavonic uliji, Lithuanian aulys "beehive" (hollow trunk), Armenian yli "pregnant").
alveolus al·ve·o·lus (āl-vē'ə-ləs)
n. pl. al·ve·o·li (-lī')
A small angular cavity or pit, such as a tooth socket or an air sac.