The word "most toughened" refers to something that has become the hardest or most resilient version of itself. Some possible antonyms for this term could include "weakened," "softened," "vulnerable," or "fragile." These words suggest a decrease in strength or durability, making the object or person described more susceptible to damage or failure. Other possible antonyms might include "unseasoned," "untested," or "untried," which suggest a lack of experience or preparation. Whatever the antonym chosen, it should be a term that conveys the opposite of the robust, battle-tested quality that "most toughened" implies.