The term "most imponderous" refers to something that is incredibly light, weightless, or lacking in any significant mass. Antonyms for this term would include "heavier", "denser", "solid", "substantial", or "weighty". These antonyms are often used to describe objects or materials that have a greater degree of physical mass or are capable of exerting force or pressure. For example, a lead brick would be considered an antonym to "most imponderous" due to its heavy weight and density. Similarly, a mountain range or a large building would be considered an antonym to this term due to their immense size and solid structure.