“Mind’s P’s and Q’s.”
Be on your best behaviour and be careful of your language.
There are various proposed explanations of this. Amongst the most plausible are the notion that ale used to be ordered in either pints or quarts and you needed to be careful which you were given. The fact that typesetters needed to be careful when setting type because the 'p’s and 'q’s looked similar seems a better explanation.
(courtesy of phrases.org.uk )
I will agree with what Mr. Micawber said, and add that the correct way to say the expression is, “Mind your P’s and Q’s.”
I always thought the expression came from children’s problems learning to write in school. As Mr. Micawber says, lower-case P’s and Q’s are very similar, and smaller children may confuse them.
A similar expression is, “Make sure your I’s are dotted and your T’s are crossed.”
That means, “Make sure everything is done completely and is in perfect order.”
This expression definitely comes from writing practice in school.