What is Insignificant Other?
1.
Someone you are casually dating, while seeing others on the side. This person is almost always someone you would never consider dating exclusively, but, you keep around as a backup plan for free Saturday nights.
Maria: Oo, the new guy in Calculus is cute!
Virginia: Wait, I thought you and James were dating?
Maria: No, no, we're just insignificant others.
See
2.
Friend with benefits.
Sophie: Are you and Brian officially a couple?
Nadia: No, we just hook up occasionally. He's my insignificant other.
See
3.
Someone you are casually dating, while seeing others on the side. This person is almost always someone you would never consider dating exclusively, but, you keep around as a backup plan for free Saturday nights.
Maria: Oo, the new guy in Calculus is cute!
Virginia: Wait, I thought you and James were dating?
Maria: No, no, we're just insignificant others.
See