Unbreakable is a perk from Van Buren, the canceled Fallout 3 project by Black Isle, as well as in J.E. Sawyer's Fallout Role-Playing Game.

You see dead people, swing away, and no one calls you Mr. Glass. How's that for a pop-culture reference, jackass? Critical hits against your head, arms, legs, torso, and groin have to do 50% of your total hit points to achieve a break.

Robert Briggs would have this perk. It gave him a remarkable ability to dodge crippling bones, or it might just have given him a skeleton of adamantium.