When we go to visit, do remember to mind your ps and qs, children — we don't want another incident like last time.
Etymology
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.
But in the early 17th century, there is the expression pee and kew in the sense of "highest quality, best possible" (Rowlands' Knave of Harts of 1612: “Bring in a quart of Maligo, right true: And looke, you Rogue, that it be Pee and Kew.”; also "in thy Pee and Kue" 1602).