One who grints; a person who uses the dialectal form 'grint' or an agent noun form meaning 'one who grins.'
From the dialectal verb 'grint' or as a back-formation agent noun with the '-er' suffix (meaning 'one who does the action'), following standard English patterns like 'runner,' 'teacher,' 'baker.'
The '-er' suffix is one of the most productive word-building tools in English—it turns almost any action into 'someone who does that action,' which is why we can invent new words like 'grinter' intuitively!
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