Most marshmallow refers to something that is soft, fluffy, and light. Antonyms for this term can include words that convey hardness, heaviness, and density. Some of the best antonyms for most marshmallow include words like solid, hard, dense, heavy, tough, and rigid. These words describe things that are stiff, tough to manipulate, and lack the lightness and softness of marshmallows. For instance, a brick is hard, heavy and solid, and very different from a marshmallow, which is soft and fluffy. Similarly, an iron anvil is dense and rigid, while a marshmallow is light and airy.