Who was the first to "mind his P's and Q's?"
1st origin of Mind Your P's and Q's.
There are few of us who at one time or another have not been admonished to "mind our P's and Q's," or in other words, to behave our best. 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 talley 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 and "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."
A second origin story comes from early printing presses.
Printers placed individual letters on a typeset to print a page of text. The letters were reversed, making it easy to mistake lowercase p's and q's in setting the type. A reminder to stay watchful of the details could have come from this time as well. In a similar setting, this expression has been attributed as an adage for teaching children to spell.
Ps and Qs are the plural form of the letters P and Q. Among grammarians there is some disagreement about how to spell Ps and Qs - either upper-case or lower-case and either with or without a hyphen. You may see the phrase as mind your p's and q's or mind your Ps and Qs or (occasionally) mind your P's and Q's or (rarely) as mind your ps and qs.
Meaning:
Be on your best behavior; be careful of your language.