1712, "belonging to a prehistoric age," see old + world. Meaning "of or pertaining to Eurasia and Africa," as opposed to the Americas, is by 1877. The noun phrase Old World in this sense is by 1590s. The division of the earth into Old World and New World among Europeans dates to 1503 and Italian explorer Amerigo Vespucci's use of Latin Mundus Novus for the lands of the western hemisphere found by Columbus and others, indicating they were not part of Asia.
The Known World is usually divided into four Parts, Europe, Asia, Africk and America. But it is a most unequal Division, and I think it more rational to divide it thus. Viz. the Known World, first into two Parts, the Old and the New World; then the Old World into three, Europe, Asia, and Africa; and the New into two, the Northern and Southern America. [Guy Miege, "A New Cosmography, or Survey of the Whole World," London, 1682]
attested by 1898 as a speculation.
If through fear of entangling alliances the United States should return the Philippines to Spain, Mr. Page asserted that the predatory nations would swoop down upon them and a world war would result. [New York Times, Dec. 16, 1898]
Applied to the first one almost as soon as it began in 1914 ("England has Thrown Lot with France in World War" — headline, Pittsburgh Press, Aug. 2, 1914). World War I was coined 1939, replacing Great War as the most common name for it; First World War, World War II, and Second World War all also are from 1939. Old English had woruldgewinn, woruldgefeoht, both of which might be translated "world war," but with "world" in the sense of "earthly, secular."
by 1884, from French mal du sìecle, "world-weariness, atrophy of the spirit, aristocratic boredom, deep melancholy over the condition of the world," supposedly a characteristic condition of young romantics in Europe in the early 19c. It answers to German Weltschmerz.