The quality of being rough or having an uneven, coarse surface; also means harshness in manner or tone.
From Middle English 'rough' (possibly from Old Norse 'rúfr' meaning scabby) plus the suffix '-ness' which creates abstract nouns.
Roughness at the microscopic scale is why sandpaper works—you're not really filing things smooth, you're creating millions of tiny rough edges that scrape away material. Roughness fighting roughness!
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