A person or device that churns, especially one who makes butter; a machine used for churning cream or milk.
From 'churn' plus the suffix '-er' (indicating a person or thing that performs an action), a productive suffix creating agent nouns since Old English.
The '-er' suffix might be English's most productive affix—we can turn almost any verb into a person-noun: 'teacher,' 'baker,' 'runner,' 'churner'—and it's been working since before written English!
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