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.