The quality or state of being down, low, or in a downward direction; a feeling of sadness or depression.
From Middle English 'down' (from Old English 'dūn', meaning hill or downward) plus the suffix '-ness' (from Old English, used to form abstract nouns indicating a state or condition). The sense of being low or sad developed from the physical downward direction.
The suffix '-ness' is one of English's most productive word-building tools—it can attach to almost any adjective to make a noun representing that quality. 'Downness' is relatively rare because we usually say 'sadness' or 'lowness' instead, but it's perfectly valid!
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