What is the X in X-mas?
early 14c., "female teacher, governess," from Old French maistresse "mistress (lover); housekeeper; governess, female teacher" (Modern French maîtresse), fem. of maistre "master" (see master (n.)). Sense of "a woman who employs others or has authority over servants" is from early 15c. Sense of "kept woman of a married man" is from early 15c.