Callipygian is a word that is rarely used in everyday conversation, and it means having well-shaped buttocks. While it might be fun to know this word, it is also essential to know the antonyms of callipygian. Some of the antonyms of callipygian are flat, unshapely, and shapeless. These words are pretty much self-explanatory and mean the opposite of having well-shaped buttocks. Knowing the antonyms of callipygian is important because it helps to improve one's vocabulary and comprehension skills. In summary, the antonyms of callipygian are flat, unshapely, and shapeless.