A person who complies; someone who obeys rules, requests, or demands.
From comply (from Old French complir, Latin complere 'to complete, fulfill') plus agent noun suffix -er. The meaning evolved from 'completing' a request to 'obeying' or 'following' requirements.
Interestingly, 'complier' exists but is rarely used—we prefer 'someone who complies' instead. This shows English's aversion to creating agent nouns from certain verbs, especially those related to attitudes or behaviors rather than trades or professions.
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