In a manner that inspires fear or dread; in a way that is frightening or terrible.
From 'fearsome' plus the adverbial suffix '-ly.' 'Fearsome' itself comes from 'fear' plus the adjective-forming suffix '-some,' meaning 'full of' or 'characterized by.'
Medieval writers loved 'fearsome' when describing dragons and giants—it has a literary quality that pure 'frightening' lacks, which is why fantasy authors still instinctively reach for it when they want to evoke old storybook atmospheres.
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