fancy woman       
noun [ C ] UK old-fashioned uk/ˈfæn.si ˌwʊm.ən/ us/ˈfæn.si ˌwʊm.ən/
 情人
the woman that a person is having a sexual relationship with, but is not married to
  "They all think I'm his new fancy woman," she confided. “他们都认为我是他的新宠”,她坦白地说。 
 - More examples
 - I saw his fancy woman by chance one day.
 - Grandpa left Grandma and ran off to Australia with his fancy woman.
 - Why don't people like my son-in-law and his fancy woman realise the misery the internet can cause?
 - He actually brought her here. His bit on the side. His fancy woman.