Where did the Expression "mind your P's and Q's" come from?

Answer 1

I like this theory best:

"There have been several theories posted on Phrase Finder about "minding your P's and Q's." To mind your Ps and Qs is to be careful; cautious. The Ps here are said to be pints and the Qs to be quarts. The publican "chalks up" or "puts on the slate" the drinks supplied to customers; they must be aware of how much they have drunk or their bills will be unexpectedly large."

For more, read here:
http://www.phrases.org.uk/bulletin_board/13/messages/1416.html

Answer 2

This answer came from word-detective.com:
"As to where "mind your P's and Q's," meaning "be very careful" or "behave yourself" came from, I'm afraid that there is no clear answer, though folks have been saying it since the late 1700's. The consolation is that there are a number of fascinating theories, so you can pretty much take your pick of the following.

One theory is that the phrase comes from the practice in certain British pubs of tallying a customer's purchases on a blackboard behind the bar, with the notation "P" standing for "pints" and "Q" for quarts. If a customer failed to pay close attention and "mind his P's and Q's," he might well find by evening's end that the barkeep had padded his tab.

Another theory, drawn from the schoolroom, is that any child approaching the mystery of penmanship soon discovers that the lowercase "p" is devilishly easy to confuse with the lowercase "q." Thus, the theory goes, generations of teachers exhorting their small charges to "mind your P's and Q's" created a enduring metaphor for being attentive and careful. A similar theory centers on typesetters in old-fashioned printing shops, where the danger of confusing lowercase "p" and "q" was increased because typesetters had to view the typeset text backwards.

Still other theories tie the "P" to "pea" cloth (the rough fabric used in "pea jackets") and the "Q" to "queue," which meant a ponytail, either that of the fancy wigs worn by courtiers of the day or the real ponytails commonly worn by sailors. In the upscale version of this theory, young aristocrats were cautioned not to get the powder from their wigs on their jackets made of pea cloth. The sailor version has old salts advising newcomers to dip their ponytails in tar (a common practice, believe it or not), but to avoid soiling their pea jackets with the tar."

Answer 3

I read somewhere recently, can't remember where, that it comes from the fact that brawls used to start among sailors in taverns, and the bartenders would yell, "Mind your pints and quarts!" as in, "Get back to your drinks and stop fighting!"

Answer 4

Mind your "Parents and Questions" ? I had an Uncle that told me that once. Where it came from, is beyond me.

Answer 5

No one knows for sure, but here's a great list of the theories:

http://www.everything2.com/index.pl?node_id=1361596

Answer 6

p's = please

Q's = Thank you

in other words be polite