watch one's p's and q's

noun pl. P's, p's

1. the sixteenth letter of the English alphabet: from the Greek pi, a borrowing from the Phoenician
2. any of the speech sounds that this letter represents, as, in English, the (p) of peace
3. a type or impression for p or P
4. the sixteenth in a sequence or group
5. an object shaped like

adjective

1. of p or P
2. sixteenth in a sequence or group
3. shaped like