Underivable means not able to be derived or not having a source of origin. So, the antonyms for this word should be the opposite meaning or a word that means derived or having a source of origin. Some of the antonyms for underivable are derivable, derivational, sourced, originated, deducible, attributable, inferable, etc. These words indicate that something can be traced back to its origin or source. For example, if a theory is derivable, it means that it can be deduced logically from a set of principles. Therefore, using antonyms like derivable or sourced can help to convey the opposite meaning of underivable in communication.