Definition of French leave in English:

French leave

noun

mass noundated, informal
  • Absence from work or duty without permission.

    ‘the caretaker had taken French leave’
    • ‘I wouldn't get too worked up about this, you might have to take French leave to sort it out.’
    • ‘I'm beginning to think you took a French leave from the Rangers.’

Origin

Mid 18th century: said to derive from the French custom of leaving a dinner or ball without saying goodbye to the host or hostess. The phrase was first recorded shortly after the Seven Years War; the equivalent French expression is filer à l'Anglaise, literally ‘to escape in the style of the English’.