What the heck does "Mind your Ps and Qs." mean?

Maybe they look so much alike when printed, it is a way of saying PAY ATTENTION to details.  

(?) I made it up.    p and  q   see what I mean?  This post was edited by WM BARR . =ABSOLUTE TRASH at June 12, 2019 1:56 PM MDT

https://en.wikipedia.org/wiki/Mind_your_Ps_and_Qs

It means in pig English, be careful of what you say...don't swear at a judge or belittle him...if you offend him you will be In trouble

Pints and quarts.   It specifically refers to alcohol consumption.  As in don't become belligerent. This post was edited by Benedict Arnold at June 12, 2019 6:12 PM MDT

No one really knows.
There are three guesses.

One theory alleges that it was a warning to French gentlemen learning formal dancing in the 18th century to mind where they placed their feet (pieds) and watch that the pigtails on their wigs didn’t flick across the faces of other dancers.

A second theory posits that it was a warning to type-setters when setting the lower-case letters p and q. These letters, when back-to-front in printers’ cases of lead type, were difficult to tell apart. The wrong choice might produce an embarrassing misprint on the page.

Most likely is that Ps and Qs are “pleases and thankyous”; in other words, it’s a warning (particularly to children) to be polite when visiting someone else’s home or entertaining them in your own.