"Spirit of the staircase" or "Staircase inspiration"This phrase is a famous allusion to the witty remarks one thinks of when it is too late, as when one is leaving a meeting and going down the stairs. Paradoxe sur le Comédien (1773-1777)
I never have wit until I am below stairs. Jean de La Bruyère, according to J. J. Rousseau. Esprit de l'escalier, backstair wit, is credited to M. de Treville by Pierre Nicole. For use of this phrase see The King's English, p. 32. Note.