I can't tell how serious you're being but I read a really good book on this subject- The History of White People

The TL;DR on that is that whiteness is a social category, not an objective observation of human beings and their differences. For most of American history, as an example, Anglo-Saxons, Dutch/Low Germans and Scandinavians were considered a superior race to the 'alpine' and 'mediterranean' races of High Germans, Spaniards, and Italians. Irish weren't Anglo-Saxon, they were Celtic and were thus considered inferior. The racism people observe when they see 'Irish need not apply' signs or slurs directed at Italians in the 1800s were because those people were not considered 'white' at the time. It's an over-simplification, but these groups needed to be incorporated into the dominant group before they would be given the treatment we generally think is normal for white people.

Which is very jarring to us, since obviously Irish and Italians and Bavarian Germans are 'white'. But it literally does vary, and the entire purpose of the category is to render people inside of it superior by virtue of belonging to it, it's a category that exists to express supremacy.

"white" is a concept made up to justify slavery and white supremacy. you are actually hitting on something really important, the definition of being white HAS varied widely in history. when the concept of whiteness was first being developed, it was in order to justify categorizing people as white or non-white, and then the eugenics movement ran away with that concept to promote racial supremacy- not racial identity, but racial supremacy. so "white" as a category did indeed refer to specifically condoned peoples with supposed genetic, moral, intellectual superiority. and the definition of "white" did indeed vary then as well as now. irish, italian, slav, spanish, jews were all once distinctly non-white by the definition of "whites" at one point. the definition has changed since then because it has always been a non-scientific concept designed to identify "us" from "them" and justify the subjugation of whoever was considered non-white at the time.

Oh so the definition of being white varies now? People are dermofluid or something? "I'm white skinned but I'm not white."

The definition of being white, like all definitions, can indeed vary based on time and place, yes. Whether someone is white "enough" to be included in the category of "white people" is not an objective fact and will change from culture to culture, from time to time, and even from one individual perspective to the next.

Racial categories in Europe were more complex than they generally are today, especially in America. It's difficult to maintain distinctions in race between different European nationalities when everyone's immigrating to the same place and having kids together, so over time these subtle distinctions have dropped off somewhat in favor of the simpler categories of "white" and "non-white." But some of these distinctions still remain, for instance, many people who identify as "white nationalists" or even "white supremacists" also hate Jews, including Jewish people with white skin. Hitler's infamous 14 words declare that a future must be secured "for our white children," yet clearly he did not consider white-skinned Jewish people to be included in that definition.

As absurd as it may be to say that someone can have white skin but not be considered white, it can happen. The reason it doesn't make sense is because race is, to a large degree, something that is socially constructed and nonsensical.

Imagine considering Xi Jinping Winnie the Pooh memes to be racist, while simultaneously arguing that you can't be racist against white people.

And for good measure, considering the use of the word "crazy" to be an offensive term against the mentally ill.

Those things are crossing the line. But overt racism is fine as long as it's self-deprecating.