Ps and Qs are for "pints and quarts" when drinking in a pub. You "mind your ps and qs" so you don't drink too much.
This thread has me thinking about other old sayings. My nana used to say "Ditto, cabbage head!", which could mean either "back atcha" or "I agree." If it was really dark, it was "black as your hat", and when talking about someone she didn't recognize, "I didn't know him from Adam's off ox."