Ostheer translates literally to "East(ern) Army" or "Army of the East" (Ostarmee or Ostheer). The Germans, especially the military, are famous for their compound words; Falschirmjagergewehrzweiundvierzig (Paratrooper rifle 42, or FG42), Panzerabwehrkanone (tank/armor defense gun/cannon, which you know as PaK, or PaK 38 in-game) and so on are a couple of examples. Even their military nomanclature is outrageously overcomplicated by Western standards: 16.RFSS-PzG-Div/16.SS-Pz-Aufklärungs-Abt. SdKfz 234 'Puma' Schwere Panzerspähwagen translates directly(ish) to "16th SS Panzer Reconnaissance Abteilung (Battalion), 16th SS Division Das Reich Sdkfz 234 'Puma' Heavy Armored Scout Car". Mouthfull, eh?