What does "mind your p's and q's" mean and where did it originate?
There are multiple delightful and unproven theories about the origin of the phrase Mind your p's and q's. My favorite is that bartenders used to shout it during fights to tell people to watch their pints and quarts, but the Oxford English Dictionary says that particular origin story “can be neither substantiated or dismissed.”
All the reference books I checked say that the origin of the phrase is unknown. The earliest uses aren’t plural and don’t give any hint about the origin. Pee and kew simply meant “high quality.” For example, here’s a sentence from the year 1612:
Bring in a quart of Maligo, right true: And look, you Rogue, that it be Pee and Kew.
It could also mean “at your best.” Here’s an example from 1607:
At her p. and q. neither Marchantes Daughter, Aldermans Wife, young country Gentlewoman, nor Courtiers Mistris, can match her.
Nobody knows where we get the phrase Mind your p’s and q’s.
I’ve heard people say that it means “mind your pleases and your thank yous” as something an adult would say to a child or that it relates to choreographed dances that were performed in the French court, but there is no evidence to support these origins. Nor is there any evidence to back up that it comes from telling children to avoid mixing up the letters P and Q when they are learning to write or that it comes from typesetters mixing up the letter.
How to Write P’s and Q’s
If you’ve ever been to one of my book signings, it’s very likely that I wrote “Mind your p’s and q’s,” in your book. I think it’s a fun phrase, and I love that it uses an exception: The exception is that you normally don’t use apostrophes to make things plural, but you do use apostrophes to make single letters plural such as P and Q. The apostrophe is especially important when you are writing about A’s, I’s, and U’s because without the apostrophe, readers could easily think you are writing the words as, is, and us, but the rule applies to all single letters. You can write the letters themselves either uppercase or lowercase. That part is a style choice.
Ammer, C. “Mind one’s p’s and q’s.” The American Heritage Dictionary of Idioms. Houghton Mifflin Harcourt: New York. 2013. p. 295.
“P’s and Q’s, n.” Oxford English Dictionary, online edition. http://j.mp/1Jfgdd7 (subscription required, accessed June 1, 2015).