The quality or state of being fairish; moderate or mediocre quality.
From 'fairish' (adjective) plus the nominalization suffix '-ness' (from Old English 'ness,' originally meaning 'headland' but used to form abstract nouns), transforming an adjective into a noun representing the abstract quality.
The '-ness' suffix is one of English's oldest word-building tools, used since Old English times to convert almost any adjective into an abstract noun—'fairishness' lets us talk about the quality itself rather than just calling something fairish.
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