Comes from the early pub days when beer and ale was served in pint and quart containers. The tab was kept on a chalkboard used to count the pints and quarts consumed. To watch your Ps and Qs is to control your alcoholic intake and behavior.
Not only did pub keepers maintain the count of pints and quarts consumed, they often maintained a tab for regular customers, especially sailors. The sailors tab was sometimes paid directly out of the sailors pay by the ship's captain. This to assure the pub keeper of payment. However, this created the opportunity for the pub keeper to charge for a few extra pints and quarts. And in some cases the captain was in on this little deception, and shared in the extra payment. Hence it was to the sailors best interest to keep count of the pints and quarts. To mind his Ps and Qs.
officially, no, I never read on the jumpseat.
