- (transitive, ergative) To make something hard or harder (tough, resistant to pressure).
- (intransitive) To become hard (tough, resistant to pressure).
- (transitive, dated) To become or make a person or thing resistant or less sensitive.
"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"