I find it intriguing that this word has such disparate definitions.

tartar: 1. Dentistry A hard yellowish deposit on the teeth, consisting of organic secretions and food particles deposited in various salts, such as calcium carbonate. Also called calculus. 2. A reddish acid compound, chiefly potassium bitartrate, found in the juice of grapes and deposited on the sides of casks during winemaking.  

ETYMOLOGY: Middle English tartre, potassium bitartrate, from Old French, from Medieval Latin tartarum, argol, from Medieval Greek tartaron.  

Tartar: 1. also Ta·tar (  tätr) A member of any of the Turkic and Mongolian peoples of central Asia who invaded western Asia and eastern Europe in the Middle Ages. 2. Variant of Tatar (sense 1, 2). 3. often tartar A person regarded as ferocious or violent.  

IDIOM: catch a Tartar; To grapple with an unexpectedly formidable opponent.  
ETYMOLOGY: Middle English Tartre, from Old French Tartare, from Medieval Latin Tartarus, alteration (influenced by Latin Tartarus, Tartarus) of Persian Ttr; see Tatar.  

source > The American Heritage® Dictionary of the English Language: Fourth Edition.  2000.

Of course there’s the Tartarus of Greek mythology, that looks as if it spawned everything that followed… the word, not the place.