I am shocked! I have always heard the phrase of "mind your Ps and Qs" came from the UK.
-Mind your pints and quarts. This is suggested as deriving from the practice of chalking up a tally of drinks in English pubs (on the slate). Publicans had to make sure to mark up the quart drinks as distinct from the pint drinks.-
From what I understand, Quarts are for the men...
I was always taught it was to have good manners or behave, as in "mind your pleases and thank yous", Ps andQs.
I prefer your explanation and know a quart is two pints, as in quarter of a gallon but our gallons are bigger than yours

Thank you! As I mind my Ps andQs