1820, "mistress," from Medieval Latin hetaera, from Greek hetaira "female companion," fem. of hetairos "comrade, companion, good friend," from PIE *swet-aro-, suffixed form of root *s(w)e-, pronoun of the third person and reflexive (see idiom). Classical plural would be hetaerae or herairai.
Typically a slave or foreign woman devoted to private or public entertainment. In Athens, where citizens could legally marry only daughters of full citizens, opposed to "lawful wife," and thus embracing everything from "concubine" to "courtesan."