Disputed; see the alt.english.usage FAQ. May be an alteration of "mind your 'pleases and 'thank yous". Other possibilities include: when behaving poorly in a pub due to drinking too much beer, the bartender may use the phrase meaning "Be mindful of how many pints and quarts you are drinking because it is starting to show in your behavior."; or in typesetting where letters are arranged in mirror orientation and a lowercase p and q can be easily confused.
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