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to threaten; to menace; to intimidate
to startle; to frighten; to scare
「脅かす」 has another reading with different meanings.
彼は私の安全を脅かすような発言をしたんだよ。
He made a statement that threatened my safety.
環境汚染が野生動物の生存を脅かしている状況だ。
Environmental pollution is threatening the survival of wildlife.
経済危機が多くの企業の存続を脅かしているね。
The economic crisis is threatening the survival of many companies.