edition of Collins English Dictionary: an alteration of “Mind

your ‘please’s and ‘thank you’s”.

Pints & Quarts & advice for typesetters & schoolchildren are also popular theories…

http://geekswithblogs.net/mtreadwell/archive/2004/04/03/3642.aspx

http://www.yaelf.com/aueFAQ/mifmindyourpsandqs.shtml

http://www.idiomsite.com/psandqs.htm

http://www.phrases.org.uk/meanings/248000.html

http://www.word-detective.com/back-f.html

http://www.straightdope.com/classics/a3_264.html

According to my most reliable source, the pub theory (watch the host when he chalks up the Pints and Quarts you have drunk on credit, so you won’t be overcharged) is the one most often cited, but there is no evidence that it is actually the source of the expression. Most likely, it comes from schoolrooms, when teachers told their students to make sure they understood which way around a “p” and a “q” (which look just alike, but face in opposite directions) would be. So, when kids learned to write, they were told to mind their Ps and Qs because it was easy to get them confused.

A similar theory I have read about is that it came from typesetting, back in the days when typesetters had to put each individual letter in a rack to set type. Because the letters appeared backwards when they were set (so they would print right side around, like a mirror image), it was really easy to confuse the lower case p and q because they were mirror images of each other. So, typesetting masters would tell their apprentices to mind their Ps and Qs, meaning not to get their lower-case p and q mixed up.

Look: pq they are the same but backwards.