be on (one's) good behavior(redirected from be on their good behavior)
be on (one's) good behavior
To be very polite or exercising exceptionally good manners. Primarily heard in US. Now, you kids must be on your good behavior for your Auntie this weekend! Good morning, class. I hope everyone was on their good behavior for the substitute teacher yesterday.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.