Filthy is dirty pushed to the extreme, dirt you can see and smell and feel on your fingers. An oven nobody has touched since spring, a football kit after a wet match, a window with years of city grime baked onto it.
It is far stronger than dirty and it always sounds disgusted. It is also the word for rude in a sexual way: a filthy joke, filthy language, a filthy magazine under the bed. British English stretches it further, so a filthy night means rain coming at you sideways, and a person in a filthy mood is best left alone. 'Filthy rich' means rich to the point of being offensive about it. The noun behind all of this is filth, which covers both the dirt and the rude material.
Filthy is filth with a -y stuck on the end, and it has looked that way since Middle English, where it was written filthi. Filth is the noun for the dirt itself, and the ending is the ordinary English one that also gives dirty, sticky and greasy. Nothing exotic happened to this word; it stayed exactly where it was.
Middle English
filthy
English, today
filthy
From Middle English filthy, filthi, equivalent to filth + y.
Etymology from Wiktionary, CC BY-SA