The quality or state of being clownish; foolish or crude behavior, mannerisms, or characteristics; buffoonery as a personal trait.
From clownish + -ness (a suffix that creates abstract nouns from adjectives, indicating a quality or state). Standard English word formation, similar to 'foolishness,' 'crudeness,' or 'silliness.'
The '-ness' suffix is the English language's favorite tool for turning qualities into things we can discuss—'clownishness' lets us talk about foolishness as if it's an actual substance someone possesses.
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