Fig.: to be careful about one's behavior; to be very careful about what one says or does

Your father's boss is coming for dinner tonight, so you'd better mind your P's ans Q's. Your dad's promotion is at stake

Questa sera abbiamo a cena il capo di vostro padre, perciò farete meglio a comportarvi bene perché c'è in ballo la promozione di papà

Since his serious accident due to drink-and-driving, Tom has been minding his P's and Q's

Tommaso ha avuto un grave incidente per guida in stato di ebbrezza ed ora sta bene attento a quello che fa

Your grandmother is coming for dinner tonight, so you had better mind your P's ans Q's. You know how set she is on good manners

Questa sera viene la nonna a cena, perciò state bene attenti a come vi comportate: lo sapete che ci tiene molto alle buone maniere

The original meaning of this expression is in doubt and many different explanations have been suggested about its origin; here are some of them: 1) In the early pub days, beer and ale were served in pint and quart containers. The tab was kept on a chalkboard used to count the pints and quarts consumed. To watch your P's and Q's was to control your alcoholic intake, along with your check! 2) A teacher's warning to keep handwriting legible by distinguishing between the lower-case letters p and q. 3) Advice to printer's apprentices to avoid confusing the backward-facing metal type lowercase p's and q's. 4) From mind your Please's and Thankyou's