In a manner that causes harm, injury, or damage; in a way that is hurtful or damaging.
From harmful (harm + -ful adjective suffix) + -ly adverb suffix. Both suffixes are Old English in origin and represent core English word-building patterns.
The adverb 'harmfully' appears frequently in modern scientific and medical writing—food additives are tested to see if they 'harmfully interact' with other substances—showing how the word has become specialized in technical discourse where precision about causation matters greatly.
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