"Do you even own pants? Oh, wait. You do. I remember now." - Azriel
Question #297 posted on 07/23/2003 midnight
Q:

Dear 100 Hour Board,
Where does the phrase "Mind your P's and Q's" come from?
- PQ

A: Dear PQ,
There are several theories on this.
1. When customers ordered ale in the pubs, their order was denoted as either "p" (pint) or "q" (quart). They needed to "mind" their "p's and q's" to be sure that their order wasn't mixed up with someone else's.
2. "P and q" meant "prime quality."
3. "P and q" stood for "please" and "thank-you's."
4. P's and q's referred to the difficulty children had in learning to write--they would mix up their p's and q's.
5. P's and q's referred to printers' need to watch the type carefully when they set it, as the letters were backwards, so it was easy to mix up p and q.

m-w.com claims the fourth theory. OED lists the various meanings and suggested origins but basically says no one knows for sure. You can believe whatever makes you happiest.
Cheers,
- Ambrosia