Staircase wit is a calque (or loan translation) of German Treppenwitz which in turn is a calque of French esprit d'escalier. I recently bought a humorous 19th century history book called Treppenwitz der Geschichte (Staircase wit of History), written in German by William Lewis Hertslet (German-born Briton). It has been expanded by later authors, and I am slowly reading it.

Ceci n'est pas un seing.