You have a very thick skull. The first time you take this perk it reduces the chances of you getting knocked unconscious by 50% (subsequently by 25%).”— Fallout Tactics in-game description
With this perk, the chance of getting knocked unconscious is reduced by 50%.
J.E. Sawyer's Fallout Role-Playing GameEdit
You've been banged around enough that you've developed a very thick skull. Critical hits against your head have to exceed 25% of your total hit points to achieve a maim, and over 75% to achieve a break. BONUS: When combined with Unbreakable, this changes to 50% for a head maim, with no chance of a head break.