"The Great War" -- as, until the fall of France, the British continued to call the First World War in order to avoid admitting to themselves that they were now again engaged in a war of the same magnitude. [Arnold Toynbee, "Experiences," 1969]
Also formerly with a verb form, Old English greatian, Middle English greaten "to become larger, increase, grow; become visibly pregnant," which became archaic after 17c.