1. to become hard or firm, or to make something hard or firm
2. to become less sympathetic or more determined, or to make someone this way
3. to become accustomed to difficult conditions

Examples

• The concrete will harden within 24 hours.
• Cold weather hardens the ground.
• His expression hardened when he heard the news.
• Years of criticism had hardened her heart.
• The soldiers were hardened by months of training.
• Living in the mountains hardened him to harsh weather.
• The clay hardens when it dries.