heterogeneous people of eastern Burma, 1759 (as Carian), from Burmese ka-reng "wild, dirty, low-caste man" [OED].
fem. proper name, Danish shortened form of Katherine. Rare before 1928; a top-10 name for girls born in the U.S. 1951-1968.
The modern pejorative use in reference to a person regarded as ignorant, meddlesome, entitled, racist, or generally negative, is attested by 2005, originally often with reference to meanness or stupidity, and exploded in popularity 2018, with more emphasis on the racism and privilege. Its use as rhetorical shorthand probably was encouraged, if not inspired by, the 2004 movie "Mean Girls" (screenplay by Tina Fey) and by comedian U.S. Dane Cook's 2005 stand-up act, both of which produced memes and Twitter references. Claims that it originated in African-American circles are unsupported.
Beth Harpaz's 2001 book "Girls in the Van," about Hillary Clinton's U.S. Senate campaign, reports that Clinton's assistants Karen Dunn and Karen Finney were known as The Karens. Finney went on to a career as a commentator, and some of the earliest abstract uses of Karen in the late 2000s are as the personification of a liberal do-gooder.