The phrase “to mind one’s p’s and q’s” means to take pains; to be careful and precise. More conjectures have been advanced to explain the original meaning of this phrase than upon any other equally obscure.
Each has a certain degree of plausibility. The simplest explanation is that it was an incessant admonition among pedagogs to their young charges, warning them to note the right-handed knob of the p and the left-handed knob of the q.
But if such admonitions were given to youngsters just learning to print the alphabet, why was there not a like warning to mind their b’s and d’s in which the knobs are also reversed?
Another, of the same category, is that it was a warning to young apprentice printers who might be readily confused in picking out type, because the face of a type letter is just the reverse of the printed character. But here, again, the explanation is weak because the reverse of p is d, not q.
Another, and more likely explanation is that the expression originated in the old inn or alehouse. A customer, bent upon a convivial evening, would have his accounts chalked up against his final reckoning, so many pints (p’s), so many quarts (q’s). A little carelessness on the part of the barmaid might spoil his whole evening.
But other less plebeian explanations have been offered, dating back to the courtly etiquette of the seventeenth and eighteenth centuries when men wore queues.
One of these refers to the probably frantic efforts of French dancing masters to instruct young gentlemen in the stately steps and deep courtesies of the minuette. The young men had indeed to mind their pieds and queues (feet and pigtails) to avoid loss of balance and to keep the pigtail from bobbing over the head or to lose entirely the huge artificial periwig.
Another associates p with the old word “pee,” a kind of coat worn by men in the fifteenth to seventeenth centuries, now surviving only in the word “peajacket.” This account would have it that “to mind your pees and queues” was a wifely admonition to avoid soiling the jacket from the grease or flour of the queue or pigtail.
This explanation seems the least likely of the various ones that ardent delvers have offered.