This idiom originated in the Spring and Autumn Period, when monarchs of various countries competed for hegemony. During the Warring States period, the goal of Qin was clear, that is, to destroy the other six countries and unify the whole country. However, the goal of the other six countries was still to dominate the whole country, thinking only about immediate interests and dreaming meaningless dreams in the Spring and Autumn Period, which led to the dream of the Spring and Autumn Period.
3. So when we see someone with unrealistic ideas, we will say, "Dream your big dream in the Spring and Autumn Period!" , showing disdain and denial of this person's ideas.