A hard, crusty deposit that forms on teeth, or historically, a fierce warrior from Central Asia (Tatar).
The dental meaning comes from Medieval Latin 'tartarum' (possibly from Arabic or Persian), while the warrior meaning derives from Tatar peoples; the spellings merged because of similar pronunciation and both senses coexisted in European languages.
Medieval people were so scared of Tatar warriors that their name became synonymous with harsh, crusty deposits—it's like the word absorbed the fear of something rough and unstoppable!
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