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to put on airs; to act big; to throw one's weight about; to be overbearing; to be domineering; to be bossy; to be pushy; to be proud; to be haughty; to be arrogant; to swagger; to boast; to brag
彼は成功してから威張るようになった。
He became arrogant after succeeding.
何の実績もないのに威張っている人は嫌われやすい。
People who act arrogant are easily disliked.
威張らずに謙虚でいることが大切だ。
Staying humble and not boasting is what matters.