A person who contravenes or violates a law, rule, or agreement; someone who acts contrary to established rules.
From contravene plus -er (agent suffix). This legal term designates the actor who violates rules—the parallel to 'offender' in English law.
The word 'contravener' has a very formal, legal ring to it—you'll hear it in court cases and legal documents rather than in everyday conversation, which makes it sound more serious than just 'rule-breaker.'
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