A person who agrees; one who accepts a proposal, condition, or statement.
From agree + -er (suffix forming agent nouns meaning 'one who does'). The -er suffix is fundamentally Germanic and appears throughout English vocabulary.
While grammatically sound, 'agreer' is extraordinarily rare in actual English use. We prefer 'someone who agrees' instead. These kinds of logical-but-unused words show the limits of English productivity—not every morphologically correct combination actually survives in speech.
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