A person who grins; someone who smiles broadly, or an animal that bares its teeth.
From 'grin' (Old English 'grennan') plus the agent suffix '-er' meaning 'one who does' the action. The agent suffix '-er' creates nouns for people who perform actions.
English loves creating agent nouns with '-er'—a 'runner' runs, a 'singer' sings, a 'grinner' grins—and we can do this spontaneously with almost any verb, which is why English feels so flexible.
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