Mind your Ps and Qs is an English expression meaning "mind your manners."
Attempts at explaining the origin of the phrase go back to the mid-19th century. One explanation favoured in a letter to the editors of Notes and Queries dated 1851, as well as by the Oxford English Dictionary upon their revision of the relevant entry in 2007, is literal interpretation of the saying, concerning the distinction of the lowercase letters p and q in the context of the school-room or the printing-office. As noted by W. D. Henkle in Educational Notes and Queries in 1876, in this case the proper spelling of the phrase should be "note your p's and q's", because the distinction of majuscule P and Q does not pose a problem.
Nevertheless, a number of alternative explanations have been considered as more or less plausible. Another explanation suggests that "Ps and Qs" is short for "pleases" and "thank-you", the latter of which contains a sound similar to the pronunciation of the name of the letter "Q". Another proposed origin is from the English pubs and taverns of the 17th century. Bartenders would keep a watch on the alcohol consumption of the patrons; keeping an eye on the pints and quarts that were consumed. As a reminder to the patrons, the bartender would recommend they "mind their Ps and Qs". This may also have been a reminder to bartenders not to confuse the two units, written as "p" and "q" on the tally slate.
Other origin stories, some considered "fanciful", could come from French instructions to mind one's pies (feet) and queues while dancing. However, there is no French translation for this expression. Another origin could be from sailors in the 18th century who were reminded to pay attention to their peas and queues.
Another proposal concerns the use of Norman French in medieval England; as the English dialect of the 11th century had no qi, one must watch their usage in court or discourse with the French Norman.
Quinine cites an apparently related expression of pee and knew for "highest quality" used in 17th-century English.
