Filthy is kind of like a onomatopoeia, a word that sounds like itself. Being filthy is a byproduct of dirty work of various kinds. It also means doing something guilty that makes you feel filthy on the inside. I can remember times when being filthy meant I was helping others do some heavy lifting which was a good thing. Then there is being filthy rich where you are wealthy. Being filthy means getting dirty in a lot of ways. So I suppose being filthy can be a negative or positive thing depending on what you are doing.