A borrowing from French, the phrase esprit d’escalier, also esprit de l’escalier and esprit des escaliers, literally wit of (the) staircase(s), denotes a retort or remark that occurs to a person after the opportunity to make it has passed.

This phrase originally referred to a witty remark coming to mind on the stairs leading away from a social gathering. The image may have originated in Paradoxe sur le comédien (Paradox of the actor), an essay on theatre by the French philosopher, critic and encyclopaedist Denis Diderot (1713-1784):
—The relevant passage is as follows in Paradoxe sur le comédien. Ouvrage posthume de Diderot (Paris: A. Sautelet et C^ie, 1830):

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.
     translation:
The man of sensitivity, like me, wrapped up in what is objected to him, loses his head, and only pulls himself together at the bottom of the staircase.

In Etiquette: the American code of manners: a study of the usages, laws, and observances which govern intercourse in the best circles of American society (New York: George Routledge & Sons, [1884]), the U.S. author Mary Elizabeth Wilson Sherwood (1826-1903) had in mind a different staircase; she wrote of:

that belated wit which the French call “l’esprit de l’escalier”—the wit of the stair-case—the good things which we remember that we might have said as we go upstairs to bed after the party is over.

The earliest occurrences that I have found of the phrase in texts written in French are as follows, in chronological order:

1-: From Mémoires et voyages du prince Puckler Muskau. Lettres posthumes sur l’Angleterre, l’Irlande, la France, la Hollande et l’Allemagne (Paris: H. Fournier jeune, 1832)—the translation, by Jean Cohen (1781-1848), of Briefe eines Verstorbenen (1831), by the German nobleman Hermann Ludwig Heinrich von Pückler-Muskau (1785-1871):

Mais à présent il ne reste à nous autres Allemands, qu’un esprit à faire valoir dans la société, celui que les Français ont si bien dénommés [sic] l’esprit des escaliers, parce qu’il ne nous inspire que sur l’escalier ce que nous aurions dû dire dans le salon.
     translation:
But for now we Germans have nothing left, but one kind of wit to exploit in society, that which the French have so aptly named l’esprit des escaliers, because it is only on the staircase that it inspires us what we should have said in the salon.

2-: From Le Corsaire (Paris, France) of 29^th April 1849—the following is about the Swiss-born French philosopher, novelist and political theorist Jean-Jacques Rousseau (1712-1778):

Jean-Jacques, lorsqu’il conversait dans un salon, n’avait jamais la réplique ; jamais il ne pouvait trouver d’arguments ad hominem, de ces réponses qui vous callent un homme. Ce n’était qu’une fois sorti, sur l’escalier, que tout son esprit et toute sa verve lui revenaient, et alors il trouvait réponse à tout ; aussi tous ses amis, quand ils parlaient de lui, disaient :
— Ah ! celui-là par exemple, il avait l’esprit de l’escalier.
     translation:
Jean-Jacques, when he was conversing in a salon, never had the retort; never could he find any ad-hominem arguments, any of those replies that reduce a man to silence. It was only once he had left, on the staircase, that all his wit and all his eloquence came back to him, and then he found an answer to everything; so that all his friends, when they were talking about him, would say:
— Ah! this one really, he had l’esprit de l’escalier.

The earliest occurrences that I have found of the phrase in texts written in English are as follows, in chronological order:

1-: From a letter dated London, 19^th January 1827, published in Tour in Germany, Holland and England, in the years 1826, 1827, & 1828; with remarks on the manners and customs of the inhabitants, and anecdotes of distinguished public characters. In a series of letters. By a German prince (London: Effingham Wilson, 1832)—the translation, by the British translator Sarah Austin (née Taylor – 1793-1867), of Briefe eines Verstorbenen (1831), by the German nobleman Hermann Ludwig Heinrich von Pückler-Muskau (1785-1871):

As it is, we Germans have nothing left in society, but that sort of talent which the French call ‘l’esprit des escaliers’;—that, namely, which suggests to a man as he is going down stairs, the clever things he might have said in the ‘salon’.

2-: From Music. As it exists ‘under the Reformation.’, by ‘J. Oldschool’, published in The Knickerbocker, or New-York Monthly Magazine (New York City, New York, USA) of August 1834:

How many times have I repaired to a soirée, with a fixed determination to blame, and remained to praise! And how often have I felt—as, with the sound of “sweet jargoning” in my ears, I passed into the street—that emotion which an ingenious French writer terms l’esprit des escaliers—that is, a sort of late remorse, which suggests to a man as he is going down stairs, the things he might have said, and which he ought to have said, in the salon.

3-: From The Newark Bee (Newark, Nottinghamshire, England) of May 1838:

L’Esprit de l’escalier.—That which suggests on the stairs, the good things one might have said in the saloon.—Tour of a German Prince.

4-: From The Era (London, England) of 26^th June 1842:

The Globe newspaper, which, amid its generally trifling manner, sometimes says a smart thing, has declared Sir Richard Vyvyan to possess in a remarkable degree l’esprit de l’escalier, which means the faculty of recollecting when one is going down stairs all the sensible and witty things that one might have said before leaving the room.

The U.S. essayist and poet Ralph Waldo Emerson (1803-1882) used the expression in English Traits (Boston: Phillips, Sampson, and Company, 1856):

A slow temperament makes them less rapid and ready than other countrymen, and has given occasion to the observation, that English wit comes afterwards,—which the French denote as esprit d’escalier. This dulness makes their attachment to home, and their adherence in all foreign countries to home habits. The Englishman who visits Mount Etna, will carry his teakettle to the top.

In The King’s English (Oxford: The Clarendon Press, 1906), a book on English usage and grammar, the British authors Henry Watson Fowler (1858-1933) and Francis George Fowler (1871-1918) criticised the translation spirit of the staircase. They first gave the following quotation:

I thought afterwards, but it was the spirit of the staircase, what a pity it was that I did not stand at the door with a hat, saying, ‘Give an obol to Belisarius’.—Morley.

And they remarked:

The French have had the wit to pack into the words esprit d’escalier the common experience that one’s happiest retorts occur to one only when the chance of uttering them is gone, the door is closed, and one’s feet are on the staircase. That is well worth introducing to an English audience; the only question is whether it is of any use to translate it without explanation. No one will know what spirit of the staircase is who is not already familiar with esprit d’escalier; and even he who is may not recognize it in disguise, seeing that esprit does not mean spirit (which suggests a goblin lurking in the hall clock), but wit.

(French esprit primarily means spirit, mind, hence intelligence, wit.)

Better English renderings have been used: staircase thought, staircase afterthought and staircase wit.

In her memoirs, Scoundrel Time (Boston: Little, Brown and Company, 1976), the U.S. dramatist and screenwriter Lillian Florence Hellman (1905-1984) gave a variation on the phrase. Recollecting her experience with the House Committee on Un-American Activities, she wishes she had told the chairman, John Stephens Wood:

“There is no Communist menace in this country and you know it. You have made cowards into liars, an ugly business, and you made me write a letter in which I acknowledged your power. I should have gone into your Committee room, given my name and address, and walked out.” Many people have said they liked what I did, but I don’t much, and if I hadn’t worried about rats in jail, and such. . . . Ah, the bravery you tell yourself was possible when it’s all over, the bravery of the staircase.