Assuming power or authority without justification; arrogantly domineering and overbearing in manner.
From Latin 'imperiosus,' from 'imperium' (command, empire). Originally described the legitimate authority of Roman emperors, but evolved to describe unjustified authoritarian behavior.
Think of an emperor (imperium) who's gotten too big for his crown - imperious people act like they rule an empire even when they don't! They issue commands like royalty.
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