https://www.dictionary.com/e/slang/karen/
Karen joins a trend on the internet in the 2010s of using a first name
to make fun of certain kinds of people. A Becky, for example, is a
stereotype for a “basic” young, white woman, while a Chad, in other
corners of the internet, stands in for a cocky, young “dudebro.”
But, why the name Karen? Karen has widely been credited to Black Twitter
in the 2010s. Another suggestion is that it comes from a 2005 bit by
Dane Cook called “The Friend Nobody Likes.” (The friend was named
Karen.) An additional explanation is that it comes from the character
Karen in the 2004 film Mean Girls, who’s the subject of the popular
quote: “Oh my God, Karen, you can’t just ask someone why they’re white.”
It’s even been put forth that Karen comes from the even earlier 1990
film Goodfellas, in which one of the characters is named Karen.
Whatever the origin of the slang, the name Karen, apparently, is
popularly thought of as a generic-seeming name for a middle-aged white
woman of a certain generation. According to Social Security data, Karen
was indeed the fourth most popular name for newborn girls in the 1960s,
peaking at #3 in 1965.
Record of the insult Karen appears as early as September 2016 when a
Tumblr user, joematar, made fun of a promo for Nintendo Switch in which
a white woman (appearing to be in her late 20s or early 30s) brings the
gaming device to a party. The user refers to this supposed kill-joy as
Karen: “Oh shit, Karen brought her stupid Nintendo thing to the party
again. We’re DRINKING, Karen. We’re having CONVERSATIONS.”
The character was further developed in December 2017 thanks to a
subreddit dedicated to mocking the imagined Karen (somewhat like Cook’s
“The Friend Nobody Likes” bit). Tropes that developed about Karen here
were that she is an annoying (and always annoyed) middle-aged, suburban,
minivan-driving white, divorced mother of poorly behaved boys (of whom
she has custody) who has a so-called “speak to the manager” haircut.