A person who asserts or claims something together with another person.
From 'coassert' plus the agent suffix '-er,' which turns verbs into 'one who does that action.' The suffix '-er' comes from Old English and is used to create occupational or role-based nouns.
The '-er' suffix is so powerful that English speakers automatically understand new words with it—call someone a 'googler' and everyone knows you mean 'one who googles,' showing how productive this suffix is!
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