verb
-ens; -ened; -ening
to become hard or firm or to make (something) hard or firm [no obj]
to become more definite and strongly felt or to make (something) more definite and strongly felt [no obj]
[+ obj] :to make (someone) hard :to make (someone) less emotional and less likely to feel sorry for other people
[no obj] :to begin to be or seem harsh, angry, serious, etc.
harden your heart
If you harden your heart or if something hardens your heart, you stop having kind or friendly feelings for someone.