The word "most dissolvent" is used to describe a substance that can dissolve other substances easily. When we talk about antonyms for this word, we are looking for words that mean the opposite, i.e., a substance which doesn't dissolve other substances easily. Some antonyms for the word "most dissolvent" could be insoluble, undissolvable, unmeltable, solid, and impermeable. These words describe substances that cannot be dissolved or melted easily, remain solid and intact. For example, sand is an insoluble substance, which means it will not dissolve in water or other liquids. Similarly, metals are generally insoluble and solid.