hard‧en [intransitive and transitive]
to become firm or stiff, or to make something firm or stiff [≠ soften]:
It will take about 24 hours for the glue to harden.
if your attitude hardens, or if something hardens it, you become more strict and determined and less sympathetic [≠ soften]:
Attitudes towards the terrorists have hardened even more since the attack.
if your face or voice hardens, or if something hardens it, you look or sound less sympathetic or happy [≠ soften]:
His face hardened momentarily, then he looked away.
to make yourself not feel pity or sympathy for someone