Mind your Ps and Qs means watch the details, and there are two popular explanations.
The first is that because a lower case p and q are mirror images of each other, printing presses had to pay close attention to which one they used.
The other, and more likely, explanation is that English pubs marked Ps and Qs on a blackboard to record each customer’s consumption of pints and quarts.
“Mind your Ps and Qs” meant keep an eye on your tab.
