We all get that feeling, right? Having a conversation with someone who makes you feel stupid, or useless, or wrong or you just plain old had nothing to say -- and then five minutes later you're like, "Oh! Why didn't I think of that?"
As it turns out, there's a word for it. l'esprit de l'escalier.
I guess I'll have to take up a French vocabulary. Or people can just keep Pinteresting their own -- that works, too. ;)