"Most halfgrown" refers to something that is almost fully grown but not quite there yet. Some antonyms for this phrase could include "fully grown," "mature," or "adult." Alternatively, if we focus on the "most" aspect of the phrase, we could use antonyms such as "least grown," "immature," or "undeveloped." Depending on the context of the phrase, different antonyms may be more appropriate. For example, if referring to a plant or animal, "fully grown" may be the best antonym, while if referring to a child, "immature" would be more fitting.