Ps and Qs
Opinions differ as to the origin of the phrase, 'P's and Q's'. Some say that it was once shouted in pubs when things were getting a little rowdy, "Mind your Pints and Quarts!", these being the main measurements of drinks before the Second World War. Others say that it was an old printer's axiom; a reminder to typesetters to pay attention to the details.
Regardless of its origins, this admonition has been common in post-Victorian Britain as an abbreviation of 'to mind your manners' or, more specifically, to say both 'please' ('p's) and 'thank-you' ('thank-q's).
This is inevitably a child's very first introduction to all-important manners, and parents will soon weary of the mantra-like repetition of, 'Say please/thank-you!' every few minutes for the first five, ten or fifteen years of their child's existence.
In this case, the tedium of repetition is surely justified - a child who doesn't mind their ps and qs, the most basic of good manners, is being given a very poor start in life, especially in British society, where every single social transaction is eased by reiteration of these phrases.