Toughen is a versatile word that can have many different synonyms depending on the context in which it is used. Some synonyms for "toughen" could include words like strengthen, fortify, harden, steel, bolster, and embolden. These words all have the sense of making something stronger, more resilient, or more capable of withstanding difficult circumstances. Other synonyms might include phrases like "toughen up," "buttress," "beef up," and "tough out." No matter which synonym is chosen, the idea is always the same: to make something or someone tougher and more able to handle whatever challenges come their way.