【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.