to become more severe, determined, or unpleasant(使)变严厉,(使)变坚强,(使)变冷酷
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。