This is a special perk, so it cannot be taken at new levels like other perks. It can be obtained only by using the Auto-Doc in the Vault City courtyard, then selecting the option to 'ride again'. An Intelligence below 4 is required to get it.
+2 maximum Hit Points
Trying to view this perk in the character sheet makes the game crash because its description's number points towards that of Alcohol Raised Hit Points.