The term "un sound" refers to something that is flawed, deficient or unstable. While there are several antonyms for this term that can denote different meanings, the most fitting ones are "sound," "stable," "strong," "reliable" and "robust." Something that is "sound" is free from any defect, error or weakness. It denotes the idea of something that is complete, well-structured and dependable. On the other hand, "stable" or "strong" refers to something that is solid, steady and sturdy. It suggests a sense of firmness, durability and resistance to external forces. Likewise, "reliable" or "robust" indicates something that is dependable, trustworthy and effective over a long period of time.