In a manner showing the characteristics or effects of disease; in a sickly or unhealthy way.
From diseased (from disease meaning illness + -ed) + -ly (adverb suffix). Medieval English added -ly to adjectives to turn them into adverbs describing how something happens.
Shakespeare used words like this to describe not just physical illness but moral corruption—a character might act 'diseasedly' to show their mind was twisted by greed or jealousy, not just that they had a fever.
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