mind one's ps and qs
Uncertain; OED (2007) considers the most likely origin the distinction of the minuscule letters p and q in either the school-room or the printing-office.
In the early 17th century however, there is the expression pee and kew in the sense of “highest quality, best possible”. Also recorded as in thy Pee and Kue in 1602.
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- (idiomatic) To be very careful to behave correctly.
When we go to visit, do remember to mind your ps and qs, children – we don't want another incident like last time.