tartar — as in: tartar on the teeth
It was the sort of romantic fustian we both barely tolerated, but she let it play; the hoofbeats of the Tartar
kettledrums began thudding through the room.
William Styron -- Sophie’s Choice
calcified deposits on the teeth, formed by the continuous presence of dental plaque (also called calculous)
Do not confuse this with the proper noun, Tartar, which references a central Asian people once known for their ferocity in battle.
The expression tartar sauce refers to a mayonnaise-based sauce commonly served with seafood and sometimes spelled tartare sauce. You may also see steak tartar in reference to a ground beef dish that is eaten raw; though the preferred spelling of the dish is steak tartare.
Much more rarely, tartar can reference anyone with a bad temper, or it can reference a reddish crust or sediment in wine casks.