mind one's p's and q's

mind one's p's and q's  {v. phr.}

To be very careful what you do or say; not make mistakes.

When the principal of the school visited the class the students all minded their p's and q's.

If you wish to succeed you must mind your p's and q's.

(From the old U.S. Navy when sailors marked on a board in the bar how many Pints and Quarts of liquor they had taken. It was bad manners to cheat.)

Syn.: WATCH ONE'S STEP.