A person who shows excessive reverence or worship of grammar and grammatical rules; an extreme grammarian.
From 'grammat-' (from Latin 'grammaticus') plus 'lator' (from Latin 'latores,' bearers or worshippers), or more likely '-lator' from the same root as 'idolator,' meaning 'one who worships'.
This is an absolutely hilarious insult: it means someone who treats grammar like a religion—and it shows that the tension between grammar-obsessed people and everyone else has existed for centuries!
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