"weak will" is correct and usable in written English.
You can use it to refer to someone's lack of strength, resolve, or determination. For example, "Her weak will prevented her from achieving her goals."
A weak French performance, with Poland also weak, will be exacerbated by current euro and zloty weakness, which is likely to exert further pressure on consensus pretax profit forecasts in our view.
Herein lies a major weakness of the powerful: they easily assume that the weak will give up in the face of overwhelming force.
A disease, illness, weak will or choice?
"The strong will win, and the weak will lose".
Those of you who are weak will be merged.
Women's reluctance to negotiate was never attributed to weak will or a failure of nerve.
Spending on infrastructure in areas where the private sector is weak will go ahead.
"The strong will survive and the weak will shake out," he added, "and that's actually good.
The strong will get stronger and the weak will get weaker".
For consumer publishers, then, the strong will get stronger and the weak will suffer most.
The weak will carry on being violated and exploited, their cries unheard.
