From the base of G4628; {dry} that {is} hard or tough (figuratively {harsh}severe): - {fierce} hard.
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Mat 19:8He saith unto them, Moses because of the hardness ofG4641 your heartsG4641 suffered you to put away your wives: but from the beginning it was not so.
Mar 16:14¶ Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heartG4641, because they believed not them which had seen him after he was risen.