Yes, it’s French and yes, you can also use it in English. Literally translated, “esprit d’escalier” means “staircase wit”. It describes a situation, where you only come up with a witty response to an insult (or a witty contribution to a discussion) when it’s already too late. That is, you have left the scene and are on the staircase. The German “Treppenwitz” is also derived from this French expression. “Witz” has its original meaning “witty thought”, not its later meaning “joke”.