Underset refers to the act of supporting or reinforcing something. There are various synonyms that can be used in place of the word underset, including undergird, underpin, bolster, buttress, underlie, brace, support, and prop up. Undergird means to provide a foundation or basis for an argument, while underlie refers to the foundational concepts or principles that form the basis of a particular concept. On the other hand, buttress and brace both refer to providing additional support to an object or structure. Bolster and prop up are both used to imply providing additional strength or support to something. In summary, there are plenty of synonyms that can be used to express the concept of underset, each with its own subtle difference in meaning.