The phrase "rock hard" typically refers to something that is firm, solid, and unyielding. The antonyms for this phrase are words that express the opposite meaning, such as soft, flexible, pliable, or supple. These words describe materials or objects that can be easily bent, compressed, or molded. For example, a cushion or pillow may be soft and yielding, while a rubber band or a piece of plastic can be flexible and pliable. Some other antonyms for "rock hard" include flimsy, delicate, weak, or brittle, which describe objects that are easily broken, damaged, or destroyed.