verb

1. To become hard (tough, resistant to pressure).

2. To make something hard or harder (tough, resistant to pressure).

3. To strengthen.

Synonyms: inure

adjective

1. Unfeeling or lacking emotion due to experience; callous.

   “The bloody scene could reduce even the most hardened soldier to tears.”

2. Firmly established or unlikely to change; inveterate.

3. Having extra defences against attack; highly fortified.

   “a hardened air base”