MTV uploaded this clip about the history of the word "Caucasian" from their program Decoded with Franchesca Ramsey to Facebook, and commenters had a completely civil discussion about it. Just kidding, they lost their minds. In case you can't watch the video, suffice it to say that the word Caucasian, which in the United States just means "White," refers specifically to people from the Caucasus Mountains to the east of the Black Sea. Southern Russia, Georgia, Azerbaijan, and eastern Turkey.
How did this come to mean "all white Europeans"? Basically, some very unscientific scientists decided that the people there were the sexiest, and had the best skulls and therefore must be where Adam and Eve came from (seriously) and that Europeans were descended from them and everyone else was an evil, bestial corruption. This was the heyday of social Darwinism (aka scientific racism), and the slave-owning and later Jim-Crowing US was glad to incorporate these notions into the law, since it basically justified everything.
The final transformation came by via the Supreme Court, who informed an Indian man who wanted to immigrate to the US that he, despite the beliefs of contemporary experts, did not count as Caucasian because only white people were Caucasian. It's also worth noting that the term "Aryan" comes from Sanskrit texts about ancient Indians, so that idea was not unusual.
So, how did this rather dry history lesson go over in the comments? Badly. The implication that people in the past were racist (spoiler alert: people in the past were very racist. So, so, so racist. Even today's racists would be like "ok, buddy, calm down.") seemed to many people to be an anti-white crusade. Many people clearly didn't even watch the video.
Never mind that people during these time periods also thought that novels would melt women's fragile little brains or that the human body would fall apart if subjected to speeds over 30mph on a train—somehow calling 18th and 19th century folks ignorant is an insult to humans of today.
And it goes on like that for thousands and thousands of responses. Well, at least historians of the future will have so, so much more to choose from when studying how stupid we were. If you're actually interested in the topic, though, you should check out The History Of White People. Trigger warning: it's written by a black woman.