Definition of esprit de l'escalier in English:

noun

• Used to refer to the fact that a witty remark or retort often comes to mind after the opportunity to make it has passed.

  ‘I am frequently afflicted with esprit de l'escalier.’

  ‘Now if I'd managed to say all that off the cuff then I would be a genius comedian, but as it was I had to make do with experiencing esprit de l'escalier on the way home.’