Historically, it was when oneliterally 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.
The relationship of intercourse stemming from the possibly of getting lucky ending with herpes. Done by pursing lips together like a fish and tonguing as far into a rotten snatch as possible.
im gonna fish lick a bitch tonight chances are im gonnaget herpes.