The term "be substantive" means to be meaningful or significant, but its antonyms refer to shallowness or insignificance. Words like trivial, unimportant, and frivolous are antonyms of being substantive. Other antonyms include immaterial, superficial, and insignificant. When something is not substantive, it lacks content, depth, or relevance. One can use antonyms such as insubstantial, inconsequential, and lightweight to describe things that are not substantive. These words highlight the lack of importance or strength in a particular context or situation. Thus, when looking for ways to describe something that lacks value or significance, antonyms for "be substantive" will convey the right message.