What does it mean to "watch your p's and q's?"
A type setting term. Back when they used engraved type letters bonded on little blocks of wood; type was 'backwards'. Also a term used for places that sold pints and quarts of beer--which is more relevant to cooking.
Also an admonition to young British children to mind their manners - their 'pease' and (thank)cues.
Also to "mind your pints and quarts," meaning to watch how much beer or ale you're drinking in a pub. The bartenders of 18th century (or so) England would keep track of how many pints or quarts a patron was drinking, so they'd know how much to charge at the end of the night--as well as when to cut off the rowdy drinkers.