What is the X in X-mas?
"chaste, pure, virgin," 1590s, originally (early 15c.) "belonging to or dedicated to Vesta," Roman goddess of hearth and home. The noun is recorded from 1570s, short for Vestal virgin, one of four (later six) priestesses (Latin virgines Vestales) in charge of the sacred fire in the temple of Vesta in Rome. The goddess name, attested in English from late 14c., corresponds to, and may be cognate with, Greek Hestia, from hestia "hearth," from PIE root *wes- "to dwell, stay" (cf. Sanskrit vasati "stays, dwells," Gothic wisan, Old English, Old High German wesan "to be").