c.1369 (implied in Tartary, "the land of the Tartars"), from M.L. Tartarus, from Pers. Tatar, first used 13c. in reference to the hordes of Ghengis Khan (1202-1227), said ult. to be from Tata, a name of the Mongols for themselves. Form in European languages probably influenced by L. Tartarus "hell" [e.g.
letter of St. Louis of France, 1270: "In the present danger of the Tartars either we shall push them back into the Tartarus whence they are come, or they will bring us all into heaven"]. The historical word for what now are called in ethnological works Tatars. A Turkic people, their native region was east of the Caspian Sea. Ghengis' horde was a mix of Tatars. Mongols, Turks, etc. Used figuratively for "savage, rough, irascible person" (1663); Byron's tartarly (1821) is a nonce-word. To catch a Tartar "get hold of what cannot be controlled" is recorded from 1663; original sense not preserved, but probably from some military story similar to the old battlefield joke:
Irish soldier (shouting from within the brush): I've captured one of the enemy.
Captain: Excellent! Bring him here.
Soldier: He won't come.
Captain: Well, then, you come here.
Soldier: I would, but he won't let me.
Tartar sauce is first recorded 1855, from Fr. sauce tartare.