The quality or state of being fashionable; attention to or concern with fashion and style.
From 'fashion' plus '-ous' plus '-ness' (the suffix that makes abstract nouns), following patterns like 'happiness' or 'sadness'. This is an obsolete or very rare formation.
By adding '-ous' then '-ness,' speakers were creating the quality noun—but English prefers 'fashion' or 'fashionability,' showing how some word formations succeed while others fade away.
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