Filthy is a dirty word. Sure, you can say it aloud in public, but it means "dirty" — like your dirty laundry pile by the end of the week.
The joy of filthy really lies in the limitless range of its usage. Just about anything can be filthy, not just your dirty socks. You can have filthy living conditions, or wear filthy clothes, but it's also used to refer to something morally or ethically odious, too: "filthy liar," "filthy language," and so on. It can also indicate envy or outrage on the part of the user, as in our favorite use of the word, "filthy rich." Now that's a use of filthy we'd be happy to put up with.