Skinny is one of Fallout GURPS disadvantages. See: Fallout GURPS.
You are notably underweight. After figuring your height, take average weight for that height and cut it by 1/3. You may not take Handsome or Very Handsome appearance, and your HT may not be more than 14. Normal clothes and armor will not fit you, and you will be at -2 to Disguise, or to Shadowing if you are trying to follow someone in a crowd.”— Steve Jackson Games GURPS sources
With the abandonment of GURPS, Skinny was removed from the game