missus (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.

1580s, abbreviation of mistress (q.v.), originally in all uses of that word. Prefixed to the name of a married woman by 1610s. The plural Mmes. is an abbreviation of French mesdames, plural of madame, used in English to serve as the plural of Mrs., which is lacking. Pronunciation "missis" was considered vulgar at least into 18c. (compare missus). The Mrs. "one's wife" is from 1920.