There is, I believe, a French terminology that describes a missed opportunity to make a verbal response, & if the very moment is missed, the comment would be futile. Please help..........
errr...that second answer doesn't mean anything, really....Even said correctly (le moment est pass�), it's not actually an "expression" as such, just something you might say.
I'd go with jno's answer, can't think of anything else right now!
It is 'l'esprit d'escalier' (staircase wit ) , when you miss the opportunity to make a witty or briliant reply because it only comes to you whilst leaving, probably as you are still turning over the past event in your mind! The expression was coined by Denis Diderot, an C18 French writer, when writing about some of the paradoxes of humour; that's certainly one, that our wittiest remark seems only to come to us after the event.