Weak Will is one of Fallout GURPS disadvantages. See: Fallout GURPS.
You are easily persuaded, frightened, bullied, coerced, tempted and so on. For every level taken, your IQ is effectively reduced by 1 whenever you make a Will roll, including attempts to resist distraction, seduction, torture, mind control and so on. Weak Will also affects all attempts to master phobias and avoid giving in to mental problems.”— Steve Jackson Games GURPS sources
With the abandonment of GURPS, Weak Will was removed from the game