What is the X in X-mas?
c.1200, "virtuous, pure from unlawful sexual intercourse" (as defined by the Church), from Old French chaste "morally pure" (12c.), from Latin castus "clean, pure, morally pure" (see caste). Transferred sense of "sexually pure" is by 15c., perhaps by influence of chastity, though chaste as a noun meaning "virgin person" is recorded from early 14c. Related: Chastely.