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. During a dinner at the home of statesman Jacques Necker, a remark was made to Diderot 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, such as myself, overwhelmed by the argument levelled against him, becomes confused and can only think clearly again [when he reaches] the bottom of the stairs”).
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 on the étage noble, one floor above the ground floor. To have reached the bottom of the stairs means to have definitively left the gathering.
Source: Ultra Facts