Harden (verb)

1. To make or become hard or harder.
2. To become inured or habituated to something.
3. To make or become more resistant to something.
4. To make or become more unfeeling or callous.
5. To make or become more resistant to change.

Origin:

Middle english hardnen, from old english heardnian; akin to old high german hardjan to make hard.

Examples:

1. The clay hardened in the sun.
2. His heart hardened against her.
3. The steel is hardened by heating and quenching.
4. His attitude hardened with age.
5. The economy has hardened since the crisis.