Fithly is a is a term used to describe things that considered cool by some people, usually people on the dole, or in a lower income bracket. That clearly shows they do not aren't very stylish. Some things include; cheap cars with body kits, mullets, wearing motocross jerseys in town, bad makeup and doing burnouts frequent.
Definition #2 (Often used by the lower class people described in definition #1)
Fithly means that something is good, or desirable.
Something that is disgusting, that you never want to come in contact with.
Using the example of two friends, Bob and Cam.
Bob: Look at that car coming down the street.
Cam: That car is so filthy
Bob: Ha ha, sure it.
Bob: Travis Pastrana's double backflip is awesome!
Cam: His Motorbike is so Filthy!
Bob: I just watched 2 Girls 1 Cup
Cam: How can you watch that, it is so filthy
Bob: Yeah, I get nightmares about it