Learner's definition of HARDEN
to become hard or firm or to make (something) hard or firm
to become more definite and strongly felt or to make (something) more definite and strongly felt
to make (someone) hard
to make (someone) less emotional and less likely to feel sorry for other people
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.