[Northkeep] Where did "Piss Poor" (and other sayings) come from?

[Jennifer Carlson]

There are linguistic scholars who have researched (and pulled a lot of their hair out, no doubt) trying to find the origins of phrases.  There are many, many colorful "origins" floating around, as has been discussed recently.  If you remember the "bring home the bacon" discussion, you know some origins can be documented, some cannot.  Google "origins of phrases," and you will find many websites, many of which do not agree with each other.
 
Plus, language is a fluid and shifting thing.  As an example, there is an English phrase about being "too poor to have a pot to piss in," which may or may not be related to "piss poor."
 
I've never heard of any Roman tax being paid in urine.  It would be interesting to find such a statute.  It sounds like something a pro-industry emperor might support, since urine was a source of ammonia, necessary for fulling wool (I'm not sure about its role in tanning leather - that would be a Diarmaid question).  Cash-strapped Caesars, I imagine, would be more likely to charge for the removal of it rather than accept it as currency. 
 

Talana