He who is often reproved, yet stiffens his neck,
will suddenly be broken beyond healing.
Work-hardening is what happens when a metal is bent to the point of breaking. Bending a paperclip back and forth does not weaken the metal; in fact it strengthens it beyond pliability -- which is why it breaks.
People who refuse to repent as often as they are rebuked become like work-hardened metal – strengthened to the point of irreparable brokenness.