Do you have colleagues or fellow students who behave like “mean girls,” even if they are adults? I have had a few, or several. A flutist I know took her daughter to someone’s house for a school activity, and when the mother answered the door she said, “I don’t know you, my friends don’t know who you are, NOBODY knows anything about you.” And she slammed the door on her as she was saying, “Well, I only just moved here six months ago, so why would you?” That struck me as an example of adult “mean girl” behavior. (It was in a fairly ritzy suburb.)
I had a colleague and friend of longstanding, who perhaps having heard I was displeased with her conduct and performance of something of mine, called me and told me this elaborate story of somebody else having said I did such-and-such to her to make her lose a gig, trying to turn me against other friends, involving them, all in some twisted way of getting back at me, rather than dealing directly, and it took quite some time for me to sort it out and realize it was all quite impossible, no matter how much conviction she put into saying it, and after reading about the behavior pattern, I realized she was “mean-girling” me. Yecch. She is over 40 years old, too.
I’ll leave it for someone else to define “mean girl” unless no-one wants/dares to. It’s a specific social behavior pattern common among certain types of teenage girls.