When thou goest to return into Egypt, see that thou do all those wonders before Pharaoh, which I have put in thine hand: but I will harden his heart, that he shall not let the people go. — KJV, Exodus 4:21.
If you say that someone's face or eyes harden, you mean that they suddenly look serious or angry. His smile died and the look in his face hardened
To accustom by labor or suffering to endure with constancy; to strengthen; to stiffen; to inure; also, to confirm in wickedness or shame; to make unimpressionable
When something hardens or when you harden it, it becomes stiff or firm. Mould the mixture into shape while hot, before it hardens Give the cardboard two or three coats of varnish to harden it
When an attitude or opinion hardens or is hardened, it becomes harsher, stronger, or fixed. Their action can only serve to harden the attitude of landowners The bitter split which has developed within Solidarity is likely to harden further into separation. + hardening hard·en·ing a hardening of the government's attitude towards rebellious parts of the army