The state or quality of being dead, lifeless, dull, or lacking energy or emotion.
From 'dead' (no longer living, lacking vitality) plus '-ness' (suffix forming abstract nouns). The word originally meant the literal state of death, but evolved to include metaphorical uses describing dullness, lack of emotion, or absence of energy.
The 'deadness' of someone's expression or a party's atmosphere is a poetic way to describe absence—not the presence of something bad, but the absence of life, energy, or engagement, which is psychologically quite different.
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