I visited Ireland recently and my tour guide to Belfast told me about a few expressions in the English language that have come from Ireland.
One saying he talked about is, "piss poor." As in, "I don't have any money, I'm piss poor."
In the 1500s/1600s (approx), in Medieval UK/Europe, urine was used to tan animal skins (for clothes, furnishings, etc). So, poor families would all use a pot to urinate in, and they'd sell it to people who conditioned hide. So when you were really poor, you'd piss in a pot to make money.
Then comes the expression, "doesn't have a pot to piss in" for some who is really poor. Because they can't even afford the pot!
I've been researching a lot of sayings since my revelation of the above. So many expressions come from Shakespeare. I suggest that you take your favourite sayings and idioms and find their origins and post them here! I'd love to know more 
Great thread, Elyse (calling literally lauren...).
This isn't a phrase so much as a particular word, but I'll run with this for now until something nicer pops into my head.
The term
guys derives from a plotter named
Guy Fawkes. In 1605, Fawkes - along with a small group of others - planned to assassinate
James VI and I.
Fawkes was in charge of the explosives (due to military experience), and essentially became the face of the failed assassination attempt when it was foiled. He was then hanged, drawn and quartered (actually, the etymology of this phrase might be interesting!).
To celebrate the failing of the assassination attempt and the continued survival of the King, civilians lit bonfires - a reference to Fawkes' explosives. It became a yearly custom, culminating in Guy Fawkes Night. Eventually, people starting using effigies of Guy Fawkes as part of the festivities - and these effigies were, appropriately, known as Guys.
As time went on, the term generalised to refer simply to any male - and, now, seems to be generalising even further to refer to any person.
Happy to be corrected on any of this, but that's my very basic understanding.
