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.
Ryan: Hell yea man, I'm gonna go throw a fish on that bitch.
by FishMan099 July 26, 2008