Karen is a name that comes from a meme or a movie or something, and is used in the internet to describe a woman, usually middle-aged, who is very nasty, rude, demanding, always wants to "speak with the manager" over trivial things, obnoxious, selfish, or even abusive.
When I played Operator 112 on Steam one of the callers was actually called herself Karen, and was throwing a fit at a restaurant because she claimed they poisoned her soup and want her to pay for it.
A few months ago I customized a scenario for Plague Inc. that I called "Entitled Parentism". I had originally called it "Karenism" but changed it so it would apply to both genders.