An object or person becomes hardened when it's made stronger and more resistant through some process.
When we talk about something being hardened, usually it means that it's become tougher because of heat, cold, or chemicals. Think of a knife getting harder as its metal is heated and molded - now it won't bend easily anymore! If you put food in the fridge for too long, it might get hardened - either by freezing or going bad.
The word "hardened" can also be used to describe people who have become more severe or cruel over time. For example, someone might say that a person has hardened their heart towards others after being hurt many times. In this case, the person is no longer able to feel emotions as deeply as they once did.
To become hard (tough, resistant to pressure).
Unfeeling or lacking emotion due to experience; callous.
Example: The bloody scene could reduce even the most hardened soldier to tears.
