To toughen means to make something or someone stronger, more durable, or better able to handle difficulty. When you toughen your hands by doing outdoor work, your skin develops calluses that protect against blisters. When a coach puts athletes through challenging drills, she's helping to toughen their bodies and their mental resolve.

The word works both physically and mentally. Physical training toughens muscles and builds endurance. Life experiences can toughen your character, making you more resilient when facing setbacks. A student who perseveres through a difficult math unit might find that the struggle has toughened their problem-solving skills and confidence.

You can also toughen materials: leather gets toughened through special treatment, making it last longer. Glass can be toughened through heat processing to resist shattering.

Sometimes people use toughen up as encouragement to become more resilient or less easily discouraged by hardship. The process of toughening isn't always comfortable, like when basketball practice leaves your legs sore, but it's what builds the strength and endurance you need for the actual game.