Naked Mole-Rat L'esprit d'escalier (Pt.3)

In this case, “the bottom of the stairs” refers to the architecture of the kind of hôtel particulier or mansion to which Diderot had been invited.

In such houses, the reception rooms were located on the étage noble, one floor above the ground floor, so that to have reached the bottom of the stairs means to have definitively left the gathering in question.

American English speakers sometimes also call this "elevator wit".