retrotort: n. "the word for any clever remark that comes to mind after the fact (coined by Bernard Cooper)"--Elster, Charles Harrington. There's a Word for it!: A Grandiloquent Guide to life. New York: Scribners, 1996.
I thought it would be a good good idea to cover a bit the ground surrounding mot juste and bon mot. Thanks to Kirkpatrick Sale we have the term stairwit to describe that moment at home after the party when we say, "Drat! I should have responded to Wanda's bon mot, "Envy me!" with a wink and some sort of retort, even if only: "You Wish!" Too late now. Here I am all alone: just me and my retrotort."