harden
verb [ I or T ] uk/ˈhɑː.dən/ us/ˈhɑːr.dən/
(SOLID)
（使）变硬
to become or make hard
The mixture hardens as it cools. 混合物冷却时变硬了。
It is thought that high cholesterol levels in the blood can harden the arteries (= make them thicker and stiffer, causing disease). 据信血液中的胆固醇含量高会导致动脉硬化。
(SEVERE)
（使）变严厉，（使）变坚强，（使）变冷酷
to become more severe, determined, or unpleasant
Living in the desert hardened the recruits (= made them more strong and determined). 沙漠的艰苦生活把新兵锻炼得更加坚强。
As the war progressed, attitudes on both sides hardened (= became more severe and determined). 随着战争的持续，双方的态度都变得更加强硬。
Idiom
harden your heartidiom 硬起心肠
to make yourself stop feeling kind or friendly towards someone
You've just got to harden your heart and tell him to leave. 你只有硬起心肠让他离开。