From KYM
'While it's unclear where exactly "Karen" began seeing use as a pejorative character, it perhaps started with the Oh My God, Karen, You Can’t Just Ask Someone Why They're White meme from Mean Girls. Others believe that the popularity of the name may come from the character Karen from the 1989 gangster film Goodfellas.'
'Some believe that usage of the name as a perjorative stems from the Dane Cook standup comedy routine "The Friend That Nobody Like," which first appeared on the album Retaliation, released on July 26th, 2005 and recorded 2004. In the routine, Cook says:'
So I'm hangin' out with all of my buddies, and uh, I realize something, I realize something. Think of the group of people you've known the longest in your life. Think of the group of friends you've hung out with the most, maybe you're all here tonight. And this is what I realized, I had an epiphany, and here it is, right here: There is one person, in every group of friends, that nobody fucking likes. You basically keep them there, to hate their guts. When that person is not around the rest of your little base camp, your hobby, is cutting that person down.
Example: "Karen, is always a douchebag."
Every group has a Karen and she is always a bag of douche.
And when she's not around, you just look at each other and say,
"God, Karen, she's such a douchebag!"