the.com/mistress

means A woman who is a man's long-term lover outside of marriage, typically in a relationship kept secret from his wife.

from Comes from Old French 'maistresse,' the feminine form of 'maistre' (master) — simply meaning a woman who has authority or control over something, like a female head of household. For centuries 'mistress' meant nothing scandalous at all: it was the ordinary female counterpart to 'mister,' used for a woman in charge of servants, a female teacher, or the lady of the house (hence 'Mrs.' is literally short for it). The romantic-rival meaning crept in alongside the respectable one, as 'mistress of the house' slid into 'mistress of his heart' slid into 'the other woman' — the word quietly forking in two directions until the scandalous sense elbowed the domestic one into the background.