explain

I used to think the meaning of 'P's and 'Q's are abbreviations for pints and quarts, which are liquid measures. I assumed it refered to beer and its consumption. This is most likely wrong, though it's logical.

However, it is most likely refers to manual typesetting when publishers and newspapers set their type by hand. You had to pay close attention to which direction the tail of the P and the Q are set or else you created a typo in print.

A misqlaced p would look like this typo. Puite odd, you'd agree?

It means "mind your own business/affairs" and usually it's said when a person believes that another is interfering in their affairs.