"Nah, just a dirty thirty. It's straight, though."
"Fuck yeah! Dirty 30 just doubled to left!"
The expression almost always refers to women, as men are thought to always be in this state.
"Alright Ladies, this dirty thirty is on the house!"
Leo: No, I can't drink, there are exams this week.
Jimmy: No, I mean the dirty thirty from Nafer.