Advice to a child learning its letters to be careful not to mix up the handwritten lower-case letters p and q.

Similar advice to a printer’s apprentice, for whom the backward-facing metal type letters would be especially confusing.

Jocular, or perhaps deadly serious, advice to a barman not to confuse the letters p and q on the tally slate, on which the letters stood for the pints and quarts consumed “on tick” by the patrons.

An abbreviation of mind your please’s and thank-you’s.

Instructions from a French dancing master to be sure to perform the dance figures pieds and queues accurately.

An admonishment to seamen not to soil their navy pea-jackets with their tarred queues, that is, their pigtails.

Channa

1. Don’t make a silly mistake by being careless or over-confident (since the lower cases of these letters are essentially mirror images of each other).

2. Be careful not to get into trouble over something about which you should know better.

3. Be polite and use the correct social manners and mannerisms of the place and people among whom you find yourself.

Oddly enough, “mind your P’s and Q’s” had nautical beginnings as a method of keeping books on the waterfront.

In the days of sail when Sailors were paid a pittance, seamen drank their ale in taverns whose keepers were willing to extend credit until payday. Since many salts were illiterate, keepers kept a tally of pints and quarts consumed by each Sailor on a chalkboard behind the bar. Next to each person’s name, a mark was made under “P” for pint or “Q” for quart whenever a seaman ordered another draught.

On payday, each seaman was liable for each mark next to his name, so he was forced to “mind his P’s and Q’s” or he would get into financial trouble. To ensure an accurate count by unscrupulous keepers, Sailors had to keep their wits and remain somewhat sober. Sobriety usually ensured good behavior, hence the meaning of “mind your P’s and Q’s.”

Basically it means don’t get in over your head.