There are plenty of English phrases we don’t understand. Sorry, don’t mean “English.” We mean “English English,” like in Austin Powers. Some are more or less familiar. “Bob’s your uncle,” basically meaning “Ta-da,” or “voila, all’s good!” “Knackered,” meaning exhausted. “Taking the piss,” meaning—chill—making fun of something.
But then there’s that “Mind your Ps and Qs” expression that at least we pretend to understand. It’s gotta be something about “being careful or attentive,” right? Since you’re minding something? The basic definitions involve manners. According to the Oxford Dictionaries blog, to be “on your Ps and Qs” means to be “on your best behavior.” So to “mind your Ps and Qs” would presumably mean to mind those things that determine the goodness of your behavior.
There’s also a story that says minding one’s “ps and qs” is something adults would say to children—who presumably got it wrong when trying to figure out which side of the circle to put the “tail” on.
Our favorite possibility, of course, is the one in which “minding your Ps and Qs” stands for “mind your pints and quarts,” essentially advice for any inn- or bar-keep tallying up a guest’s drinks. In the days before keeping a tab, or paying immediately as the hugely muscled bartender stares directly at you, bartenders would often keep a chalk tally of the number of pints or quarts of ale a customer had consumed.
Since a quart is two times bigger than a pint, “publicans had to make sure to mark up the quart drinks as distinct from the pint drinks.”
According to most sources, this is one of the least well-founded explanations as the basis for the phrase. Which is surprising, considering how deeply people care about how much they pay for booze compared to the relative consequences of a miswritten “q.”