Extremely frightening or horrifying in a way that makes you feel intense fear or dread.
Present participle of the phrase 'blood-curdle' (to cause blood to curdle); combines 'blood' with 'curdle' (from Middle English curdlen, related to curd formation), popularized in Gothic literature of the 1800s.
Horror writers deliberately used 'bloodcurdling' because it combines two powerful things: blood (violence, danger) and a disgusting physical reaction (like vomit), making readers feel visceral revulsion at just the words themselves.
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