A person who disagrees or objects; one who does not assent or agree.
From dis- (opposite, not) + enter/assent (from Latin assentire, meaning to agree). Historically, a 'disenter' was someone who refused to conform to established church practices, especially in England during the 1600s-1700s.
Dissenters (spelled differently but same root concept) were religious rebels in England—they disagreed with the official church and faced real persecution, making 'disenter' a word loaded with the history of standing up for your beliefs.
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