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to intimidate; to frighten; to scare
to threaten (e.g. peace); to jeopardize; to endanger; to imperil
「脅かす」 has another reading with different meanings.
暗闘で急に現れて私を脅かさないでほしいな。
Please don't frighten me by suddenly appearing in the dark.
子供を脅かして泣かせるのは良くないと思う。
I don't think it's good to scare children and make them cry.
友人がドッキリで脅かしてきて心臓が止まりそうだ。
My friend scared me with a prank and my heart almost stopped.