A person or thing that drives out, forces away, or removes something.
From 'expel' + '-er' (one who does the action). The '-er' suffix marks the agent or doer, from Latin 'expellere.'
An expeller could be a school administrator removing a student, a bouncer removing a patron, or even a machine designed to extract oil from seeds—the word doesn't care about morality, just the action!
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