mind one's ps and qs

• mind one's pees and cues, be on one's pees and cues
• mind one's peas and cues, be on one's peas and cues
• mind one's pees and kews, be on one's pees and kews
• mind one's pees and ques, be on one's pees and ques
• mind one's P's and Q's, be on one's P's and Q's
• mind one's p's and q's, be on one's p's and q's
• mind one's Ps and Qs, be on one's Ps and Qs

Uncertain; OED (3rd edition 2007) says that the origin of the phrase is unknown and states that a common suggestion is that the phrase originates with the distinction of the minuscule letters p and q in either the school-room or the printing-office, but goes on to say that the chronology of the senses argues against this since no such connotation is evident in the earliest quotations.[1]

In the early 17th century, however, there is the expression pee and kew in the sense of “highest quality, best possible”[2]. Also recorded as in thy Pee and Kue in 1602.

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mind one's ps and qs (third-person singular simple present minds one's ps and qs, present participle minding one's ps and qs, simple past and past participle minded one's ps and qs)

1. (idiomatic) To be very careful to behave correctly.

   When we go to visit, do remember to mind your ps and qs, children – we don't want another incident like last time.

• Mind your Ps and Qs on Wikipedia.Wikipedia