The opposite of softens up can be described as toughens, hardens or strengthens. When you toughen or harden something, you make it more resistant to damage or external pressure. For example, a boxer needs to toughen up their body to be able to handle punches. Similarly, when you strengthen an object or a material, you make it more durable and less likely to break or wear out easily. This can be achieved through various means such as heat treatment, chemical treatment, or physical conditioning. Thus, while softening up is critical when dealing with some materials, toughening up and strengthening are also essential in specific situations.