The phrase "most in thing" refers to the latest and most popular trend or fashion. Antonyms for this phrase could include outdated, unpopular, old-fashioned, passe, unfashionable, passe, and outmoded. These words can be used to describe something that is no longer fashionable, trendy, or commonly used. While some people may strive to be the "most in thing," others prefer to follow their own unique style and enjoy being different from the mainstream. Embracing these antonyms can help individuals express themselves authentically and stand out from the crowd.