A Dumb defense mitigating attacks. Can be provided by various game abilities or plot created scenarios. From the Accelerant core rules book:

"You are unaffected by the attack in question. This indicates a defense that is not consumable. The attack does not affect you, nor will it ever affect you."

If you hear this Verbal then it means the creature/character is utterly immune to whatever was being done to them. It won't work no matter how many times you try. This should not be confused with Resist, or other game abilities where a creature may only be able to prevent something for a limited number of times.