Sorry only theories.. At least I tried??

It has also been attributed to a warning that when you were in the pub (where your bill was jotted onto a chalk board) you should make sure that the landlord was writing his p’s and q’s carefully, because they stood for pints and quarts and if he got sloppy, your bill could be considerably higher. That may also be true, but children in school were urged to mind their p’s and q’s and they didn’t hang out in pubs so I believe the more general source was the school room and the caution to take care with what you were doing..

Still other theories tie the “P” to “pea” cloth (the rough fabric used in “pea jackets”) and the “Q” to “queue,” which meant a ponytail, either that of the fancy wigs worn by courtiers of the day or the real ponytails commonly worn by sailors. In the upscale version of this theory, young aristocrats were cautioned not to get the powder from their wigs on their jackets made of pea cloth. The sailor version has old salts advising newcomers to dip their ponytails in tar (a common practice, believe it or not), but to avoid soiling their pea jackets with the tar.

Also, when a customer wanted another drink, he would use the whistle built into the cup. Hence, “wet your whistle”.

Nowadays it means just to behave yourself and keep your cool.

thats the unit they serve drinks in. to keep them in line you would watch someones Pints and Quarts.