When someone/something is dirty in the sexual sense of the word. Not necessarily a bad thing...
Dude, there was clothes all over the room when I woke up...it was dirtyy...
I could see her tux thong through when she bent over...Dirtyy!
Dirty cum refers to when someone cums in a cup and adds alcohol, then gives it to someone to drink.
She got some dirtyy cum last night.