In a dark manner; also means in a grim, sinister, or mysterious way.
From English 'dark' (Old English 'deorc') plus the adverbial suffix '-ly.' The word has been used since medieval times, with both literal and figurative meanings developing in parallel.
The word 'darkly' is used for both physical darkness and emotional tone—'he spoke darkly'—which shows how our language treats emotional states as if they're literally shadows, a pattern called 'metaphorical language' that appears in almost every language.
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