Meaning of “harden” - Learner’s Dictionary

harden

verb [ I, T ] uk us /ˈhɑːdən/
BECOME HARD

to become hard and stiff, or to make something become hard and stiff:

This island is formed from volcanic lava that has hardened into rock.
STOP FEELING EMOTIONS

to stop feeling emotions about someone or something, so that you seem less kind, gentle, or weak:

hardened criminals

(Definition of “harden” from the Cambridge Learner’s Dictionary © Cambridge University Press)