A person who practices or advocates coercion as a method to force compliance or achieve goals.
From coercion + -ist suffix (meaning 'one who practices or advocates'). The -ist suffix comes from Latin and Greek and is productive in English for naming adherents of ideologies or practitioners of methods.
While logically formed, 'coercionist' is rarely used in modern English because most people talk about specific types of force (dictators, bullies, authoritarians) rather than using this abstract label. But it appears in historical and political theory texts when discussing governments that rely on force.
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