In French you'll hear the idiomatic "renvoyer l'ascenseur" often enough, meaning to send back the elevator, i.e., to return the favor, or in some cases to respond to a misdeed by a misdeed; but the imaginative "l'esprit de l'escalier" is a bit more arcane. Literally it means "staircase wit" and is used to describe the feeling you get when you think of a riposte only after the conversation has ended. The idiom comes from a passage in Paradoxe sur le Comédien written in 1778 by Denis Diderot, the French philosopher and writer of 18th-century encyclopedia fame, where he laments not thinking of a reply to an argument against him until he was at the bottom of the stairs.
We were left speechless, too, at the bottom of the stairs to this pretty wrought-iron gloriette, which sits high atop an artificial cave, La Grotte des Quatre Vents, in the Parc de Bagatelle in the Bois de Boulogne. Like a number of other fabriques de jardin that still exist in the park, the grotto dates to the 18th century. The gloriette was a later 19th-century afterthought.
une gloriette: usually a wrought-iron pavillon in the form of a bird cage; it can also designate a small decorative neoclassic garden pavillon
une grotte: a cave
un escalier: a staircase
l'esprit: mind; wit; spirit
une repartie: a repartee, a quick, witty reply