Is the phrase "that's mighty white of you" racist in its origins/intent?

I've heard that phrase for years and never associated it with race. I just thought white meant pure and good, or noble.
Update: I'm also quite certain I've heard it used in old films, where there would certainly have been no racism intended. I really wonder if its origins are innocent, whatever happened years later. I'd never use the phrase now for fear of offending someone.
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