To swagger or act arrogantly — to show off one's status or power and behave haughtily or bossily toward others.
あの上司は会議のときにいつも威張って話す。
That boss always speaks arrogantly during meetings.
成功してから彼は少し威張るようになった。
Since his success he has become a bit boastful.
子どもが友達に威張るので、注意した。
The child was bossing around his friends, so I warned him.
立場を利用して威張るのは良くない。
It's not good to use your position to act arrogant.