A person who disagrees; someone who holds a different opinion or refuses to agree with something.
From disagree + -er (suffix forming agent nouns). A relatively straightforward formation that gained usage in the 20th century as a way to label someone's role in a conflict.
Disagreer is delightfully direct—it's like the word committee would create if they wanted to call someone a 'non-agrees-r' without any euphemism, though we rarely actually use it because it sounds a bit judgmental.
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