"Un-trueness," as a word, can be defined as a lack of truth or accuracy. It is an uncommon word, and there are not many recognized antonyms or opposites in the English language. However, some words that can be used as antonyms of un-trueness, are truthfulness, honesty, accuracy, authenticity, veracity, and sincerity. Each of these words implies a sense of reliability, genuineness, and truth. The opposite of un-trueness can also be seen in concepts like trustworthiness, credibility, and faithfulness. These words represent the opposite of deception, falsehood, and uncertainty. In short, an antonym for un-trueness would be any of the words that represent honesty, authenticity, and reliability.