mind one's p's and q's

[mind one's p's and q's] {v. phr.} To be very careful what you door say; not make mistakes.

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

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

(From the old U.S. Navywhen sailors marked on a board in the bar how many Pints and Quarts ofliquor they had taken. It was bad manners to cheat.) Syn.: WATCH ONE'SSTEP.