Definition of be a devil! in English:

be a devil!

phrase

British
informal
  • Said when encouraging someone to do something that they are hesitating to do:

    ‘‘Go on, be a devil and stop being so staid!’’
    • ‘The nights give way to live DJs playing hip-hop etc and serious party people, so instead be a devil, go to the bar and order a tray of six shots instead!’
    • ‘But all this goodness doesn't mean you can't be a bit naughty, so go ahead, be a devil and order the individual chocolate fondue cake with vanilla ice cream.’