反动 is a politically charged adjective meaning 'reactionary' — originally a Marxist-Leninist term used to describe people or forces opposed to revolution or progress. In modern Chinese, it appears frequently in official discourse (反动势力, 反动思想). Separately, among younger speakers on social media, 反动 can be used jokingly to mean 'bad, mean' (e.g., 你好反动). Be careful: the political sense is very strong, while the slang sense is casual and not appropriate in serious contexts.