Most infantine is a term used to describe something that is very childish and immature. Some of the antonyms for this word include words like mature, sophisticated, and grown-up. Mature is an excellent antonym for most infantine, as it implies a level of knowledge, experience, and wisdom that comes with time. On the other hand, sophisticated is an antonym for most infantine that suggests refinement, elegance, and worldliness. Finally, grown-up is a direct opposite of infantine, indicating that the person or thing in question has reached or surpassed the expected level of maturity or adulthood. Each of these antonyms offers a nuanced perspective on what it means to be opposite of most infantine.