in a gruff manner; with a rough, harsh, or unfriendly tone or attitude.
Standard adverbial form from 'gruff' plus -ly, following the regular pattern for converting adjectives to adverbs in English.
The difference between 'gruffly' and 'gruffily' is subtle—both are correct, but 'gruffly' is far more common because it's simpler, showing how English speakers prefer efficiency over morphological elaboration.
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