The phrase "more in thing" refers to something that is currently popular or fashionable. Its antonyms, on the other hand, would refer to something that is either outdated or unpopular. These could include phrases such as "out of style," "unpopular," "unfashionable," or "old-fashioned." Other possible antonyms could include "uncool," "unhip," "unpopular," "antiquated," or "obsolete." It's important to note that antonyms for "more in thing" may vary depending on the context and the specific thing being referred to, as well as cultural and regional factors.