A French phrase meaning "the wit of the staircase"; a perfect witty remark, retort, or rejoinder that occurs to one after the fact or too late to be used. (Also written as "l'esprit d'escalier.") I was on the bus home long after being tongue-lashed by my boss when I thought of the perfect things to say that would take him down a few pegs. Ah, l'esprit de l'escalier!