the quality or state of being gruff; the characteristic of having a rough, unfriendly manner.
From 'gruff' plus the suffix -ness, which creates abstract nouns describing qualities or states of being.
The -ness suffix is so productive in English that speakers can attach it to almost any adjective to create a noun—you could invent 'blueness' or 'snappiness' today and everyone would understand, showing how suffix rules are wired into our language instinct.
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