The word "unstuffy" means relaxed, informal, and casual. Some of its antonyms are formal, uptight, rigid, and strict. Formal refers to something that is proper, conventional, and polished. Uptight means tense, anxious, and nervous. Rigid means inflexible, strict, and uncompromising. Strict means stern, harsh, and severe. These antonyms are often used in situations where formality, seriousness, and strict adherence to the rules are required. Unstuffy, on the other hand, is used in situations where a relaxed and informal atmosphere is desired. It is often associated with a laid-back and carefree approach to life.