A person who acts arrogantly or boastfully toward others; someone who shows off or behaves self-importantly (often used colloquially, e.g. "威張り屋さん").
あの店長はちょっと威張り屋で、部下に厳しく当たることがある。
That manager is a bit of a show-off and can be harsh with subordinates.
子どものころはクラスに威張り屋がいて、よく喧嘩になった。
When I was a child there was an arrogant kid in class and we often got into fights.
彼は自信があるように見せたくて、つい威張り屋の態度を取ってしまう。
He wants to appear confident and ends up acting like a braggart.