It may be very different in the states I have no idea but in this country it is used against women on any subject under the sun, mostly not do with discussions on race, language sometimes takes on a trajectory of it's own, so it is now often accompanied by shut up bitch. There was a brilliant article about it which discussed both sides, I wish I could remember, something about the position of white women and how sometimes they are the oppressor and sometimes the oppressed and that's why the karen discussion is so difficult.