Enkhidu said:
Actually, I think we're both right - "You" eventually did come to be used for those of higher station than yourself (for many of the same reasons for the royal "we" did - a connection with the land = need for plural pronouns), however in everyday parlance, it was always safer to assume that someone you didn't know was at least your equal (unless other, more visual cues were to be had). Therefore, "You" became an initial greeting, until a person got to know exactly who they were speaking to and what level of respect they needed to receive (you alluded to this in your second point).
Yes I think we agree. It's also worth mentioning that if we are talking about strangers meeting. This did happen very little, as most commoners in England were tied to the land in the form of the manorial system. Addressing someone as you gave the addressor time to sort out who was of higher station if that was in question. Once the station was sorted out, persons could then adjust if that was thier wish. Even so, "thee and thou" I suspect aren't overused as a Shakespearian play as other terms come in to use. i.e. cousin, good sir, master, gentle sir, ect.
Now as for other statements that go this far back.
Bring Home the Bacon:
Bacon just might be a form of payment of services rendered to a lord. Raise many pigs well and your share could be larger than most. Hence "He really knows how to bring home the bacon."
Chew the Fat:
You might be offered Pig fat as a sort of snack while you sit around talking.
Throw the Baby out with the bath water.
People didn't like to bath much as it could leave you open to getting infected. So when they did collect water and bathed, everyone did it. The water would be so black afterwards that you couldn't see the bottom of the tub and so any thing could be at the bottom, (including the baby) so you would need to check the botton before you dumped it down by the water.
Here is one that's not medieval, but might as well be. I hearthat it comes from "down under".
Three Dog Night:
A informal way of telling how cold it was.
"Last night was I had one of the dogs in bed to keep me warm, I tell ya' tonight will be a three dog night."