In a foolish, silly, or absurd manner; without sense or wisdom.
From 'daft' (foolish) + '-ly' adverbial suffix, following standard English patterns for converting adjectives to adverbs describing how an action is performed.
English is so logical with adverbs—just add '-ly' and you can turn any adjective into describing how someone did something, which is why 'daftly' instantly makes sense!
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