How about these: Dirt poor, Piss poor, and "he didn't have a pot to piss in"?
You were dirt poor if you had a dirt floor. Apparantly there was a time (somewhere) when poor people sold urine to the tallow man so he could cure his hides to sell. If you partook in this, you were piss poor. Some people who sold urine had a vessel to contain and carry the urine to the tallow man. If you didn't have such a vessel, you didn't have a pot to piss in.