The quality or state of being harmful, damaging, or having negative effects.
From deleterious + -ness, a suffix forming abstract nouns. This creates a noun from an adjective, allowing us to discuss the abstract quality of harmfulness in itself.
Adding -ness to adjectives is so productive in English that we barely notice it—but it's how we transform observations (harmful) into philosophical concepts (harmfulness) that can be debated and studied.
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