(to cause a person, liquid, determination etc) to be or become weak(er)
reduce the strength of
Cases of word usage
1) to weaken the cottage, confidence, self-discipline etc of (e.g. an army) (demoralize)
2) the deliberate damaging of machinery, material etc to weaken an opponent's activity during an industrial or political dispute (sabotage)
3) to make a hollow or tunnel under (an area of ground); to weaken (health, confidence etc) gradually (undermine)