What is the X in X-mas?
late Old English, cemes "shirt," from Old French chemise "shirt, undertunic, shift," or directly from Late Latin camisia "shirt, tunic" (Jerome; also source of Italian camicia, Spanish camisa); originally a soldier's word, probably via Gaulish, from Proto-Germanic *khamithjan (cf. Old Frisian hemethe, Old Saxon hemithi, Old English hemeðe, German hemd "shirt"), from PIE root *kem- "to cover, cloak" (cf. heaven). The French form took over after c.1200. Related: Chemisette.
loose, shirtlike garment worn by women in the European Middle Ages under their gowns (also called a chemise). The smock later became a loose, yoked, shirtlike outer garment of coarse linen, used to protect the clothes; it was worn, for example, by fieldworkers in Europe.