Ultrathin is a word that denotes something that is extremely thin, slim or narrow. Its antonyms are words that convey the opposite meaning of ultrathin, meaning something that is thick, bulky, wide or large. For example, the antonyms for ultrathin are words like bulky, thick, wide, stout, cumbersome, chubby, broad, massive, bulky, and plump, among others. These words indicate a significant contrast to ultrathin, and they are commonly used to describe objects, products and materials that are the exact opposite of ultrathin. So when you come across ultrathin-related questions or statements, you can always switch to its antonyms for clarification.