The original meaning of “expostulate” was “to discuss, examine,” but that usage is now obsolete. “Expostulate” now means “to reason with someone earnestly,” usually because you think they’re going to make a mistake. It’s not arguing just for the sake of argument; it means you’re trying to talk someone out of doing something they may regret. “Expostulate” comes from the Latin “expostulare,” meaning “to demand, dispute.”