The word "not taut" means that something is loose or slack. Therefore, some of its antonyms could be words like tight, taut, or tense. The opposite of "not taut" could be "tight," which refers to something that is held firmly, without any looseness or slackness. Another antonym for "not taut" could be "stretched," which means something is extended, pulled tight and without slack. Likewise, words like firm, rigid, or solid could be antonyms for "not taut," as they convey a sense of strength and stability. In short, "not taut" has various antonyms depending on its context, but all of them connote strictness and lack of slackness.