mind one's p's and q's

To be polite and well behaved; to mind one's manners. Be sure to mind your p's and q's when you visit your aunt this weekend! The mother reminded her young children to mind their p's and q's when they went over to their friends' houses for play dates.

Practice good manners, be precise and careful in one's behavior and speech, as in Their grandmother often told the children to mind their p's and q's. The origin of this expression, first recorded in 1779, is disputed. Among the more interesting theories advanced is that bartenders kept track of customers' consumption in terms of pints (p's) and quarts (q's) and the phrase referred to an honest accounting; that schoolchildren were taught to be careful in distinguishing the letters p and q; and that French dancing masters cautioned pupils about the correct performance of the figures pieds and queues (either abbreviated or mispronounced in English as p's and q's).

To be very particular about one’s words and/or behavior. The original meaning of p and q has been lost, and there are various theories, ranging from “pints and quarts” in the bartender’s accounts, to children confusing the two letters in learning their alphabet, to the French dancing master’s pieds and queues, figures that must be accurately performed. The term was used from the seventeenth century on but is heard less often today.