Racism refers to prejudice based on race; sexism to prejudice based on sex, &c. Classism, therefore, refers to prejudice based on social class. Is it real? Of course it’s real. But.
Regarding the working class, prejudice isn’t the most important issue. Or the second most important, or the third. You have to go pretty far down to even find prejudice on the list of things that matter.
What matters to the working class is not that it is treated as the working class, but that it is the working class. The goal is not social justice for the working class, the goal is that the class, as a class, cease to exist. That by the revolutionary act of making everyone part of the working class, no one is, and the benefits of social production be distributed evenly and fairly (no, stop right now with the bullshit about “what’s even and fair?” and “who gets to decide?” and yada yada. That isn’t the point of this post, and we can talk about it another time).
Prejudice against the worker, or against the poor, is almost a non-issue; the issue is that some people produce everything, others reap the profit from those who produce, and that this contradiction today threatens all of human civilization. The worker does not want an end to prejudice, the worker wants no longer to be “the worker.” It has very little to do with what is in someone’s head, it has everything to do with the social relations that determine all other social relations.
The term “classism” puts prejudice at the front and center of the discussion. But social classes are not caused by prejudice, rather they cause it. Class distinctions are the root cause of prejudice in the modern sense of the term (as opposed to tribal loyalty, which I would argue is a different thing). The very term “classism,” therefore, undermines this understanding, inverts the relationship, and thus makes it more difficult to understand–and therefore eliminate–class distinctions. And prejudice.