The Piss Poor
Economy.
In the olden days, they would use urine to tan animal skins. So families would all pee in a pot and then once a day it was taken and sold to the tannery. If you had to do this to survive you were "Piss Poor".
But worse than that were the really poor folk who couldn’t even afford to buy a pot. They "Didn’t have a pot to Piss in" and were the lowest of the low.
Thanks ; )
(Thanks to Thad for posting this tidbit of info on the waccoBB.net)