a. 1. Made hard, or harder, or compact; made unfeeling or callous; made obstinate or obdurate; confirmed in error or vice.
2. Rendered resistant to the effects of nearby explosions; as, a hardened missile silo; hardened warhead electronics. [ PJC ]
3. Experienced and inured to hardship; as, hardened combat troops. [ PJC ]
4. Strongly habituated to a certain type of behavior, and unlikely to change; as, a hardened criminal. Usually used only of behavior perceived negatively. [ PJC ]