Albert Barnes Commentary 2 Kings 17:14

"Notwithstanding, they would not hear, but hardened their neck, like to the neck of their fathers, who believed not in Jehovah their God." — 2 Kings 17:14 (ASV)

To “harden” or “stiffen the neck” is a common Hebrew expression that signifies unbending obstinacy and determined self-will. See the marginal references.