Someone who is imperious gives orders in a way that shows they feel superior or more important than other people. You might want the smartest kid in the class as your lab partner, but not if they have an imperious attitude and boss you around.
Near synonyms are arrogant, overbearing, and domineering. The adjective imperious is from Latin imperiōsus, from imperium "command, supreme power, empire." This Latin word is also the ultimate source of English empire, "a group of countries or territories controlled by a single ruler or one government."