The quality or state of being filthy; dirtiness, uncleanliness, or moral corruption.
From filthy (Old English fylþ + -y, meaning dirty) + -ness (Old English suffix creating abstract nouns). The -ness suffix has been used since Old English to turn adjectives into nouns describing their qualities.
Words ending in -ness are abstraction machines—they turn concrete adjectives into invisible qualities you can measure: filthy (what you see) versus filthiness (the abstract concept of being that way). This happens in thousands of English words!
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