Historically, it was when one literally threw a fish on a lady in which he is interested in. In modern times, it has been known as flopping ones penis on a lady in which he was interested in hoping for sexual recipricosity.
Mike: Hey man, look at that lady over there, she is hot!
Ryan: Hell yea man, I'm gonna go throw a fish on that bitch.
A common saying to describe someone who is down right stanky and bitchy. You can see them smoking joints, stealing your graham crackers, and pooping in your cars.
Girl 1: Did you see that girl Ashley poop on my driveway the other day?
Girl 2: Yea, what a bish fish.