A pretentious or incompetent person who shows off knowledge of grammar; a grammar pedant of low quality.
From 'grammatic' or 'grammaticus' plus the Germanic suffix '-aster' (from Old High German '-azor'), which creates a derogatory form meaning 'bad imitation of,' similar to 'poetaster' (bad poet).
This delightful insult was used to mock people who obsessed over trivial grammar rules without actually understanding language—it shows that grammarians have been annoying people for hundreds of years!
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