spoke or responded in a gruff manner; made something sound harsh or unfriendly.
Past tense of 'gruff' used as a verb, which developed from treating the adjective as an action. This shows how English flexibly converts descriptive words into action words.
In modern English, we rarely use 'gruffed' as a true past tense because 'gruff' works better as a state than an action—this is why you'd say 'spoke gruffly' instead, showing how language evolves to pick the most efficient form.
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