Definition of be on one's best behaviour in English:

be on one's best behaviour

Behave well in a social situation or when being observed: warn them to be on their best behaviour
More example sentences
  • 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.
  • Well done to all the pupils who were on their best behaviour on the night.
See parent entry: behaviour

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