In a dreadful manner; in a way that is terrible, awful, or causing fear.
From dread + -ly (adverbial suffix), though this is an archaic or rare formation. More common modern form would be 'dreadfully,' which uses the adjective dreadful + -ly.
This is a fossil word—a remnant of how Middle English speakers might have formed adverbs directly from verb stems, before standardization favored dreadful + -ly combinations.
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