side joint
A person who is the distraction of another person in a relationship. The side joint is used for comfort when the main joint or boyfriend messes up, and also for sexual relationships. Sometimes the side joint is promoted to main joint / boyfriend if there is no one in that position, or the current holder fucks up.
Angelina Jolie was Brad Pitts side joint on "Mr & Mrs. Smith" because he was currently already in a relationship.