Sometimes I've seen ド as a prefix that adds emphasis to words. So saying someone is ドバカ is saying that they are much more stupid than just バカ.
I'm wondering what the origin of ド in this context is. Does it come from 度 【ど】("degree"), as in, "the degree of X is higher"?
Also, is it always negative? I usually see it attached to negative terms, but I don't know if it's exclusively negative.
Lastly, is it impolite?