mind one's ps and qs

Meaning

Verb

• To be very careful to behave correctly.

Origin

• 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.
• In the early 17th century, however, there is the expression pee and kew in the sense of “highest quality, best possible”{{cite-book
• |year=1612
• |author=Samuel Rowlands
• |authorlink=Samuel Rowlands
• |title=Knave of Harts
• . Also recorded as in thy Pee and Kue in 1602.