L'esprit De L'escalier Meaning

(idiomatic) A conversational remark or rejoinder that only occurs to someone after the opportunity to make it has passed.

Example: 1971, Keith Thomas, Religion and the Decline of Magic, Folio Society 2012, p. 317:
  â€˜I knew not then,’ he confessed, ‘but now I think…’ It is not necessary to follow Goad along the path taken by his esprit d'escalier to see how sheer intellectual pleasure was the driving-force behind such efforts.