presumptuous
adj.behaving in a way that shows you think you are more important than you really are. You use this to describe someone who acts without asking permission or showing respect.
adj.characterised by an arrogant or disrespectful assumption of superiority or entitlement. Often describes actions that overstep social boundaries or fail to acknowledge a hierarchy.
“It was presumptuous of him to sit in the boss's chair.”
“The young assistant's presumptuous behavior during the meeting made the senior partners feel uncomfortable and unrespected.”
“The novel's protagonist is often criticized for his presumptuous belief that he can change the course of history without understanding the complex social structures he is born into.”
“However, it would have been presumptuous to use this big aircraft only for a single experiment.”
From Middle English presumptuous, borrowed from Old French presumptieus, presumpcieus, presumptuos, from Latin praesumptuosus.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.