When you expostulate, you argue strongly against someone doing something. You might expostulate with your little brother, objecting to his plan to jump off the roof into a pile of leaves.
Someone who expostulates has strong opinions about other people's plans or ideas and a desire to convince them to change their minds. You might expostulate about your town's new law banning dogs from coffee shops, arguing against it in a letter to the local newspaper. Expostulate comes from the Latin root word expostulare, meaning "to demand urgently." The English word still contains this urgency, with an added sense of persuasion and reasoned argument.