When someone is attracted to either the male or female in a coupled relationship, simply because they want what they have, or want to create jealousy in the couple.
If a person within the couple is also flirtatious with the third person, and there is a possibility of an affair, it makes them seem even more attractive to the third person.
My guy friend is always wanting me to go out to clubs with him and pretend like we are a couple to create couplenvy. It works, he always ends up with more phone numbers on those nights!
We go out together and create couplenvy, and when the other person feels like they are getting someone who is already wanted and taken, they feel like they have won the prize.