Dirty

Hi, so there's this song "Take It Off" by Kesha and the chorus goes as follows:

There's a place downtown
Where the freaks all come around
It's a hole in the wall
It's a dirty free for all

I'd like to ask, what exactly is "a dirty"? I'm assuming it's used as a noun because of the article. At first I thought she was referring to some kind of prostitute (well, not necessarily a prostitute, just a random slutty girl willing to please anyone without even getting paid) but then on second thought I started wondering if it could stand for the act of giving a handjob (you know, once the "receiver" has ejaculated, the "performer" will likely be dirty). So anyways, which is the right track to go? Neither? And yes, I get the whole idea of what she's talking about, I'm just particularly curious about that word. Maybe someone is familiar with this "slang"... I hope this is not against the rules, I suppose indecent language is allowed as long it's related to the expression in question. That said, thanks for bearing with me and thanks if anyone replies.