inamorata
A female lover or woman with whom one is in love; a mistress.
Presently Monsieur Margot made his appearance. Though very much surprised at seeing me, he did not appear the least jealous of my attentions to his inamorata.
Where it comes from
Borrowed from Italian innamorata, the feminine form of innamorato (“lover, boyfriend”), from innamorare (“to make somebody fall in love”), from in- (“in”) + amore (“love”).
Adapted from the Wiktionary entry inamorata (CC BY-SA 4.0).