Antonyms for the term "set phrase" would include words such as spontaneous, improvised, and extemporaneous. Rather than relying on pre-established phrases and expressions, these antonyms imply a more natural and off-the-cuff approach to communication. Some additional terms that might serve as antonyms for "set phrase" include unique, creative, and original, as these adjectives all suggest a departure from traditional and rigid language. By embracing different antonyms for "set phrase," individuals can expand their vocabulary and develop a more nuanced and flexible approach to language use.