The phrase "spare rod" does not have an antonym as it is not a single word but a collection of words. However, if we focus on the term "rod," the antonyms could be soft, flexible, pliable, and bendable. If we consider the term "spare," we could use antonyms such as used, required, essential, and necessary. Therefore the antonyms for "spare rod" could be required pliable, necessary soft, or essential flexible. The concept of antonyms is to provide a word that has an opposite meaning to the original word. However, in the case of "spare rod," antonyms will only apply to its individual components.