L’esprit de l’escalier
Etymology
French for "staircase wit." The philosopher Denis Diderot described being so stunned in an argument that the clever retort came to him only as he reached the bottom of the staircase, leaving. English never coined its own term, so it borrowed the French one whole (German has its own: Treppenwitz).
noun phrase · /less-PREE duh less-kal-YAY/
The perfect reply that comes to you only after the moment to say it has passed.
What it names
That universal, faintly agonizing experience of the ideal comeback arriving too late — on the stairs on your way out.
Example
Only in the taxi home did l’esprit de l’escalier strike — the reply that would have won the whole exchange.
Related idea
Sonder and saudade name feelings English lacks; this names an experience English never bothered to.