in a gruff manner; speaking or acting in a rough, unfriendly, or harsh way.
From 'gruff' plus the adverbial suffix -ly, which transforms adjectives into adverbs describing how an action is done.
The -ly ending for adverbs comes from Old English 'lich' meaning 'like,' so saying 'gruffily' originally meant 'in a gruff-like way'—every time you add -ly, you're literally saying 'like that quality.'
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