The word "up-setter" refers to someone or something that causes upset or disruption. Some synonyms for this term include agitator, disturber, troublemaker, provocateur, and rabble-rouser. An agitator is someone who deliberately causes trouble or unrest, while a disturber is someone who disrupts peace or order. A troublemaker is someone who intentionally causes problems or conflicts, often for their own amusement or benefit. A provocateur is someone who deliberately provokes or agitates others to elicit a certain reaction or outcome. Finally, a rabble-rouser is someone who incites disorder or rebellious behavior among a group of people. All of these words share the common theme of causing disruption or disturbance, with varying degrees of intent or motivation.