A person or thing that expels, drives out, or forces something or someone away.
From 'expulse' + the agent suffix '-er', meaning 'one who does the action'. Follows standard English word-formation patterns from the Latin root.
The noun 'expulser' is largely replaced by 'expeller' in modern English, but it appears throughout 18th and 19th century philosophical and medical texts referring to bodily mechanisms or enforcement officials—a reminder that older agent nouns can fade from use.
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