It is an old expression.
In olden times, poor folk used to piss in a pot and bring their piss to the leather tanner down the street who would use the liquid material to treat leather boots. He’d pay you a penny for a pot o’ piss. Even today, some leather boots in Mexico still smell like piss.
So if you were REALLY poor, you didn’t even have a pot to piss in.
There’s always something more to learn from Freepers!