The quality or state of being able to cause damage, injury, or negative effects.
From harmful (harm + -ful) plus -ness (a suffix creating abstract nouns from adjectives). Harm comes from Old English hearm meaning 'injury' or 'grief,' ultimately from Proto-Germanic origins.
The -ness suffix is one of the most productive word-building tools in English—it's so useful that speakers can attach it to almost any adjective (beautiful-ness, quiet-ness, dangerous-ness) to turn descriptions into abstract concepts.
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