hard ‧ en /ˈhɑːdn $ ˈhɑːrdn/ BrE AmE verb [intransitive and transitive]
[ Word Family: noun : ↑ hardship , ↑ hardness , ↑ hardiness ; adjective : ↑ hard , ↑ hardened , ↑ hardy ; adverb : ↑ hard , ↑ hardly ; verb : ↑ harden ]
1 . to become firm or stiff, or to make something firm or stiff OPP soften :
It will take about 24 hours for the glue to harden.
2 . if your attitude hardens, or if something hardens it, you become more strict and determined and less sympathetic OPP soften :
Attitudes towards the terrorists have hardened even more since the attack.
3 . written if your face or voice hardens, or if something hardens it, you look or sound less sympathetic or happy OPP soften :
His face hardened momentarily, then he looked away.
4 . harden your heart to make yourself not feel pity or sympathy for someone