Antonyms for the word "half grown" could include fully grown, mature, adult, developed, complete, and finished. The phrase "half grown" typically refers to a stage of growth or development where an individual or organism is only partially through their process of maturation. Therefore, antonyms would indicate a state of completion, where the organism or individual has reached full size, capacity, or maturity. For example, a fully grown tree would have a sturdy trunk, dense branches, and leaves that cover its entire frame. An adult person would have completed their growth cycle, having reached their full height and physical development. Using antonyms allows for a more accurate description of the current state of development.