A guard or keeper, especially someone who works in a prison to watch over and supervise inmates.
From Old French 'wardere' meaning one who guards, derived from 'ward' (to guard) with the agent suffix '-er' added; originated in medieval times when castle wardens kept watch over fortifications.
The term 'warder' is still commonly used in British prisons today, while Americans switched to 'guard'—it's a fascinating example of how the same job title evolved differently depending on which side of the Atlantic you're on.
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