L'esprit de l'escalier
L'esprit de l'escalier is a French expression that no French person seems to have heard of, which describes what it’s like to think of a perfect response long after the opportunity to deliver it has passed.
Etymology
Borrowed from French esprit de l’escalier (literally “mind of the staircase”), with the definite article le (“the”) at the beginning of the term elided to l’. It refers to a description of the phenomenon in the essay Paradoxe sur le comédien (Paradox of the Actor, completed 1778 and published 1830)[1] by the French encyclopedist and philosopher Denis Diderot (1713–1784). During a dinner at the home of the statesman Jacques Necker (1732–1804), Diderot was left speechless by a remark made to him. He wrote: « l’homme sensible, comme moi, tout entier à ce qu’on lui objecte, perd la tête et ne se retrouve qu’au bas de l’escalier » (“a sensitive man, such as myself, overwhelmed by the argument levelled against him, becomes confused and can only think clearly again at the bottom of the stairs”), that is, when one is already on the way out of the house.