The quality or state of being characterized by or involving adultery.
From 'adulterous' with the -ness suffix, which creates nouns from adjectives describing qualities or states. Standard pattern in English for abstracting qualities.
The -ness suffix is one of English's oldest word-building tools, used since Anglo-Saxon times. It allows us to talk about abstract concepts—'adulterousness' lets us discuss the quality itself rather than just the act.
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