I was wondering about this today, so I looked it up. Just what is tartar sauce and where did that word, "tartar" come from? I liked the explanation I found so much that I'm reposting it here:
"Okay, let's talk tartar. Or tartare. Or both. Steak tartare, as it is known today in its French appellation, was not always the, uh, gourmet meal of raw minced beefsteak mixed with egg and seasoning that it is now. As we can see in "Panetti's Extraordinary Origins of Everyday Things," this dish has its origin as a culinary practice popular in medieval times among warring Mongolian and Turkic tribes known as Tartars.
These violent fellows derived their name from the infernal abyss of Greek mythology, Tartarus. Their meal was low-quality, tough meat from Asian cattle grazing on the Russian steppe, shredded to make it more palatable and digestible. It was introduced into Germany sometime around the dawn of the 14th century, where it was prepared either raw or cooked. In fact, you get bonus points of you can guess what folks in the seaport town of Hamburg started calling it.
Tartar sauce, or as the French refer to it, sauce tartare, consists of mayonnaise, mustard, chives, chopped gherkins, and tarragon, according to C. Owen's "Choice Cooking," circa 1889. In French, it is loosely translated as 'rough,' as the Tartars were considered rough, violent, and savage. It is commonly served with fish. Yum yum.
Taken from the Straight Dope web site: "Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed. "