the quality of being gruff; a rough or unfriendly manner, especially in speech or bearing.
From 'gruff' plus the noun-forming suffix -ness, which is one of the most productive derivational morphemes in English.
English can express the same idea multiple ways—'gruffness' and 'gruffiness' both exist, but 'gruffness' is older and more standard because it's simpler, showing how English gradually settles on efficient forms.
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