According to Snopes -
http://www.snopes.com/language/phrases/wagon.asp
"As for the second set of specious etymologies put forth by the e-mail about the disposal of urine, we note that while human urine has been used in a number of cultures throughout history to tan animal skins, such tanning was typically done by families or small bands of semi-related people to process the skins of animals they'd hunted or raised themselves. Folks weren't collecting their urine, then selling it to large commercial tanneries (which used other chemical compounds in their processing of animal skins).
The phrase "piss poor" derives from the use of piss as a amplifier of the word poor, resulting in a phrase that variously means "destitute" or "of exceedingly poor workmanship or ability." (Note that in the latter instance, poor refers to a state of shoddiness rather than denoting financial poverty. A "piss poor" lawyer, for example, is one who does his job badly, not one who fails to outrun his creditors.)
Words having to do with excretory functions are routinely used in colloquialisms meant to communicate meanings of "little or no value" (e.g., "**** for brains," "not worth a fragrant fart," and "I don't give a crap"). "Piss poor" is akin to "dirt poor," with both piss and dirt serving as figurative terms for items of little worth rather than as words meant to convey literal possession or use of urine and soil. As well, the earliest known print sighting of "piss poor" dates only as far back as 1946, which also helps puts the kibosh to the notion that the term was born of the process of tanning animal hides.
By contrast, "Not having a pot to piss in" (which sometimes completes "or a window to throw it out of") does have to do with real urine, even if the phrase itself is fanciful way of saying one is really, really broke rather than a literal admission of the lack of a specific item of porcelain. Before the days of indoor plumbing, bedrooms were equipped with chamber pots, wide-mouthed vessels used by the room's occupants as ad hoc toilets during the middle of the night. (Once bodily contributed to, such containers were covered with cloths, placed back into the cabinets (commodes) they'd come from or slid under beds, then retrieved in the morning and emptied into the home's privy.) While this colorful phrase deals with a houseware item common for centuries, the saying itself dates only to 1905. However broke people may have been in the more distant past, there weren't hordes of them unable to afford vessels of any kind to pee into."
Also, Life in the 1500s
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http://www.snopes.com/language/phrases/1500.asp