mistress (n.)

c. 1300, "female teacher, governess; supervisor of novices in a convent," from Old French maistresse "mistress (lover); housekeeper; governess, female teacher" (Modern French maîtresse), fem. of maistre "master," from Latin magister "chief, head, director, teacher" (see master (n.)).

Sense of "a woman who employs others or has authority over a household and servants" is from early 15c. Meaning "woman who has mastered an art or branch of study" is from mid-15c. Sense of "kept woman of a married man" is from early 15c. As a polite form of address to a woman, mid-15c. Meaning "woman who is beloved and courted, one who has command over a lover's heart" is from c. 1500.