His heart must be very hard.

他的心肠一定很硬。

《新英汉大辞典》

Boy, thou art indeed hard of heart, if this is thy brother.

孩子，如果这是你的哥哥，你可真是铁石心肠。

youdao

She had not expected him to remember her at all and her hard little heart grew quite warm.

她完全没有指望他会记得她，她那冷酷的小心肠变得十分温暖。

youdao

In your pain and sadness, you may decide that the person you loved is hard-hearted, and in fact,has a heart of stone.

VOA: special.2010.07.18

Last week, hostility against the liquor stores on the edge of the reservation (booze is banned within it) erupted after the bodies of Ronald Hard Heart and Wilson Black Elk were found decomposing in a ditch near the stores.

ECONOMIST: Indian reservations

Only giant clinical trials measuring hard outcomes like heart attacks, strokes and deaths can tell doctors whether a heart drug's benefits outweigh its risks.

FORBES: The Statin Showdown

Many investors expect that these drugs might be approved, at least in some patients, based only on studies of their cholesterol-lowering potency, not hard proof they prevent heart attacks and strokes.

FORBES: Merck's Big Vytorin Study Could Change Rules For New Cholesterol Drugs