An insult hurled at white people who try to fight racism, or at white people who date or marry people of other races. Especially common during the Civil Rights Movement in the South, but still in use today.

As with many intended insults, it has been reclaimed, especially by Race Traitor magazine, whose motto is "Treason to whiteness is loyalty to humanity."

Here is an excerpt from their statement of belief. I am quoting them, as they can express this idea so much better than I can, and I really don't think they'll mind spreading the word. (It is a little US-centric, but understand that they are rooted in the Southern Civil Rights Movement.)

The white race is a historically constructed social formation. It consists of all those who partake of the privileges of the white skin in this society. Its most wretched members share a status higher, in certain respects, than that of the most exalted persons excluded from it, in return for which they give their support to a system that degrades them.

The key to solving the social problems of our age is to abolish the white race, which means no more and no less than abolishing the privileges of the white skin. Until that task is accomplished, even partial reform will prove elusive, because white influence permeates every issue, domestic and foreign, in U.S. society.

The existence of the white race depends on the willingness of those assigned to it to place their racial interests above class, gender, or any other interests they hold. The defection of enough of its members to make it unreliable as a predictor of behavior will lead to its collapse.