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to put oneself on guard; to stand ready; to square off
相手の攻撃に身構えた。
I braced myself against the opponent's attack.
彼は批判を予想して身構えている。
He is bracing himself in anticipation of criticism.
突然の質問に思わず身構えてしまった。
I instinctively tensed up at the sudden question.