be on (one's) best behavior(redirected from be on their best behavior)
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be on (one's) best behavior
To be very polite or exercise 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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.