The word "ungrown" refers to something that has not yet reached full maturity or development. Its antonyms, therefore, would be words that describe the opposite - things that are fully grown or developed. Some possible antonyms for "ungrown" include "mature," "fully developed," "ripe," "mellow," "seasoned," "complete," "ready," and "fully formed." These words could be used to describe a vast range of things, from food and drink to people, animals, and plants. By using these antonyms in contrast to "ungrown," we can paint a more detailed picture of the world around us and the many stages of life that things go through.