hardened

Having been accustomed or desensitized to something unpleasant through exposure or experience; inured.

hardened police officers

Unfeeling; pitiless; callous.

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

In the past, Lutao was considered somewhat mysterious, because of the national penitentiary which housed the most hardened criminals, and the restrictions on travel to the island, which have since been loosened.

Firmly established in a behavior seen as bad; unlikely to change one's ways; inveterate.

hardened criminals

Having extra defences against attack; highly fortified.

a hardened air base

a hardened server