In a way that causes fear or terror; very scary or alarming.
From 'frighten' (from Old English 'fyrhtan,' to fear) plus '-ing' (present participle) plus '-ly' (adverb suffix). The compound developed in Middle English.
Scary movies work because they trigger our ancient survival instincts—the amygdala (fear center) in our brains can't tell the difference between real danger and movie danger!
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