Definition of be on one's best behavior in US English:

be on one's best behavior


  • Behave well when being observed.

    ‘warn them to be on their best behavior’
    • ‘Barney said: ‘They really deserved to win and were on their best behaviour.’’
    • ‘But if some of the former residents were lacking in self control the children greeting the Princess were on their best behaviour.’
    • ‘I took her to the office yesterday, just to show her off; she was on her best behavior, cheerfully waving and grinning at all.’
    • ‘We all know that when the relatives get together and have a few beers the stories are funnier and more interesting than when everyone is on their best behaviour.’
    • ‘Well done to all the pupils who were on their best behaviour on the night.’
    • ‘I'll only say that everyone was on their best behavior.’
    • ‘Fans have been warned to be on their best behaviour as police have said that any trouble could lead to future exclusions.’
    • ‘Yes indeed, we'll be on our best behaviour for a week as Hilary's mum arrives tomorrow lunchtime.’
    • ‘I was told to be on my best behaviour, as to not upset Uncle Charlie.’
    • ‘But, he was on his best behaviour on the day he became an Olympian.’