Expression: Avoir l'esprit de l'escalier
Pronunciation: [ah vwar leh spree deu leh scah lyer]
Meaning: to be unable to think of witty comebacks in time
Literal translation: to have the wit of the staircase
Notes: The French expression avoir l'esprit de l'escalier refers to an inability to think of witty comebacks (or any sort of intelligent response) until it's too late to be of any use.
Esprit means wit, and the escalier, or staircase, indicates that you're on your way out of / away from the gathering where the response was needed. But you could also use this expression while still at a party; for example, if you come up with a response only after the topic has changed and it's too late for you to make your oh-so-brilliant comment without looking foolish.
Note that when used in the present tense, the expression avoir l'esprit de l'escalier refers to an ongoing affliction, indicating that the person regularly suffers from "staircase wit." To talk about a specific instance, use it in the passé composé.
Variation: avoir l'esprit d'escalier
Je n'aime pas les dîners de Matthieu, j'ai l'esprit de l'escalier.
I don't like Matthieu's dinner parties, I can never think of anything intelligent to say (until it's too late).
Céleste m'a insultée, et j'en suis restée sans voix. Le lendemain, j'ai eu l'esprit de l'escalier.
Céleste insulted me, and I was speechless. The next day, I thought of the perfect comeback.