Someone who is a close friend or roommate; a person you chum with.
From 'chum' plus the agent suffix '-er.' Directly describes someone who acts as a chum—a roommate or close companion. Common pattern in English for creating noun agents from verbs.
In British schools, your 'chummer' wasn't just a friend—it was an official social role, almost a title, showing how important shared dormitory life was to identity and belonging.
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