Harden Someone's Heart Meaning

(idiomatic, usually reflexive, dated) To make someone more resistant to something.

Example:   Seeing so much suffering for so long hardened his heart.
  She hardened her heart against his inevitable pleas.
1611, Authorized King James translation of Exodus 7:3–4:
  And I will harden Pharaoh's heart, and multiply my signs and my wonders in the land of Egypt. But Pharaoh shall not hearken unto you, that I may lay my hand upon Egypt, and bring forth mine armies, and my people the children of Israel, out of the land of Egypt by great judgments.
1998, Michael S. Ledwidge, The narrowback:
  Farrell had been to prison, and he thought the daily malignancy he had witnessed there had hardened his heart.