The opposite of "un strengthens" can be described by several antonyms. First, the word weakens represents a direct opposite of the word strengthen which means to make something stronger or more sturdy. Additionally, words such as diminish, lessen, and reduce can be used to describe the opposite of "un strengthens". These words express the idea of making something smaller, lesser, or weaker. Alternatively, terms like deteriorate, decline, and worsen show the idea of making something worse or less effective over time. Overall, there are many antonyms to describe the opposite of "un strengthens", and they all express the idea of making something less powerful or effective.