The word "unheaviest" suggests something that is not heavy. Therefore, antonyms for this word would be synonyms for "light" or "not heavy." Some possible antonyms could be "lightest," "featherweight," "insubstantial," "flimsy," "airy," "weightless," "soft," "gentle," "delicate," or "thin." These words all describe objects or experiences that are the opposite of heavy, whether in a literal or figurative sense. By using these antonyms, we can communicate a range of concepts, from the physical properties of objects to the emotional or psychological associations of weight and weightlessness.