I have always been uncomfortable with that phrase also, and I can't remember a time I used it, tho I probably did a time or two. These days my memory cells are rotting on me.
The people who fit into that category are poor as poor can be. They have more in common with their poor black neighbors than they have with middle class whites. Their racism gives them the opportunity to have someone be "less" than they are despite the fact they are in the same category economically, or even lower.
While I hate their racism I also know that they struggle daily in ways middle class whites never will. I also know that they'd give you (well, a white you) the shirt off their backs if you needed it. Funny thing about being that poor. Poor people are more generous with what they have, even though they have ever so little.
Poor white people are not "the oppressors." While their racism is abhorrent, if anyone could get them to see how much they have in common with their poor black neighbors, we'd have a revolution.