on one's best behavior

on (one's) best behavior

Being very polite or exercising exceptionally good manners. Primarily heard in US. Now, you kids must be on your best behavior for your Auntie this weekend! Good morning, class. I hope everyone was on their best behavior for the substitute teacher yesterday.
See also: behavior, on
Farlex Dictionary of Idioms.

on one's best behavior

Also, on one's good behavior. Very polite, as in Mother told the children to be on their best behavior during Grandma's visit, or The whole staff were on their good behavior while the client inspected the premises. [Late 1600s]
See also: behavior, on
The American Heritage® Dictionary of Idioms by Christine Ammer.
See also:
  • on (one's) best behavior
  • on best behavior
  • be on (one's) best behavior
  • be on (one's) good behavior
  • on (one's) good behavior
  • a whale of a
  • bundle of energy
  • think (someone) hung the moon and the stars
  • lay out the welcome mat (for one)
  • lay, put, roll, etc. out the welcome mat