Weaken, enfeeble, debilitate, undermine, sap, cripple, disable can mean to lose or cause to lose, strength, vigor, or energy.
Weaken , the most general term of this group, most frequently implies loss of the physical strength or functional efficiency characteristic of a healthy living thing or of any of its parts or loss of the soundness or stability characteristic of a strong material structure, but it may imply a loss in quality, intensity, or effective power in something material or immaterial (as by a natural or forced reduction in resources, numbers, means of support, or strengthening principle).
Enfeeble implies a more obvious and a more pitiable condition than weaken ; it suggests the state of a person greatly weakened by old age, by severe or prolonged illness, or by a state comparable to it and usually implies helplessness or powerlessness more strongly than weaken does.
Debilitate may be used in place of enfeeble but it tends to suggest a somewhat less marked and often more gradually developed impairment of strength or vitality.
Undermine and sap imply a weakening by something or someone working surreptitiously or insidiously and may further suggest a draining of strength or a caving in or breaking down.
Cripple basically means to deprive of the use of a limb; in extended use it suggests a deprivation of something causing a loss of strength or effectiveness comparable to that resulting from the loss of a limb.
Disable implies an intervention (as an event, an injury, or an influence) that deprives of strength or competence.