L'esprit de l'escalier

L'esprit de l'escalier (or l'esprit d'escalier) (staircase wit) is thinking of a clever comeback when it is too late. The phrase can be used to describe a riposte to an insult or any witty remark that comes to mind too late to be useful—after one has left the scene of the encounter. The phenomenon is usually accompanied by a feeling of regret at not having thought of it when it was most needed or suitable.

This name for the phenomenon comes from French encyclopedist and philosopher Denis Diderot's description of such a situation in his Paradoxe sur le comédien.^[2] During a dinner at the home of statesman Jacques Necker, a remark was made to him which left him speechless at the time because, he explains, 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 like me, overwhelmed by the argument levelled against him, becomes confused and can only think clearly again [when he reaches] the bottom of the stairs. “The bottom of the stairs” refers to the architecture of the kind of hôtel particulier or mansion he was invited to. In such houses, the reception rooms were located on the étage noble, the noble storey, upstairs on the French first (North American second) floor, so that to have reached the bottom of the stairs means to have definitively left the gathering in question.
Diderot's fellow-philosophe Jean-Jacques Rousseau also recognised his own affliction with l’esprit de l’escalier, staircase wit. In his autobiographical book Confessions he blamed such social blunders and missed opportunities for turning him into a misanthrope, and reassured himself that he was better at 'conversations by mail'.

Taking a red pen and underlining "..and reassured himself that he was better at 'conversations by mail'."

We are of the same misfortune, Monsieur Denis Diderot and Monsieur Jean-Jacques Rousseau! :'-)

Source: Wikipedia