Someone or something that causes extreme fear or horror, making your blood seem to 'curdle' with terror.
From 'blood-curdling' (curdle from Old English curd, soured milk) + agent suffix '-er'; the metaphor of blood curdling like milk dates to 16th-century descriptions of fear's physical effects.
The phrase 'blood curdles' isn't literally possible—blood doesn't curdle like milk—but our ancestors loved this metaphor because curdling is such a gross, irreversible transformation that it perfectly captures how fear can feel like a visceral, total takeover of your body.
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