There are various synonyms for the phrase "toughen up." Some of them include harden, strengthen, fortify, steel, and brace. When someone is told to toughen up, it usually means that they need to become more resilient and emotionally stronger. It could refer to developing a thicker skin, not taking things too personally, or learning to handle stress and adversity more effectively. People could toughen up by facing challenges head-on, learning from their mistakes, and building mental and physical toughness through exercise or other activities. Overall, to toughen up means to become stronger and more able to handle life's challenges and obstacles.