Prussia (n.)

from Medieval Latin Borussi, Prusi, Latinized forms of the native name of the Lithuanian people who lived in the bend of the Baltic before being conquered 12c. and exterminated by (mostly) German crusaders who replaced them as the inhabitants. Perhaps from Slavic *Po-Rus "(The Land) Near the Rusi" (Russians). The duchy of Prussia after union with the Mark of Brandenberg, became the core of the Prussian monarchy and later the chief state in the German Empire.