"Writers, by nature, tend to be people in whom l'esprit de l'escalier is a recurrent experience: they are always thinking of the perfect riposte after the moment for saying it has passed. So they take a few years longer and put it in print." – Louis Menand, "Bad Comma" (a review of Lynne Truss, Eats, Shoots & Leaves, The New Yorker, June 28, 2004.
I love the image of this: As you're leaving some scintillating Parisian salon, descending the grand staircase to your waiting coach outside, it hits you: you know just how you should have responded, the perfect, most brilliant retort, to the Chevalier de la Foue's insinuations about your last play (or was he perhaps referring to your relations with that promising young actor at La Comédie française?). But it is too late…
Usually de l'escalier in French, more commonly d'escalier in English.
This shows up the incompleteness of the OED (2nd ed.): the first attested use is in Zuleika Dobson in 1911, but already in 1906 the Fowlers mentioned it in The King's English and discussed the perceived fault of anglicizing it as 'spirit of the staircase'. So it must have been in use long enough to have been noticeable before that. They said 'spirit of the staircase' makes no sense unless you already know what esprit d'escalier means; if you don't, it suggests a goblin lurking in the hall clock.
No doubt the Third Edition will find antedates once they get up to E.