From FinnegansWiki
Phrase – 'mind your p's and q's', meaning 'behave correctly', 'mind your language'. In the early days of printing moveable type was set using blocks with the letters reversed. It was easy therefore to confuse p's and q's since they are mirror images of each other. it also means, 'mind your pints and quarts'. In pubs when people would start arguing, the bartenders would tell them to mind their own drinks... being pints and quarts.
