Weaken , the most general word of this group, refers to the lessening of strength or power of a person, organization or force: a political party weakened by dissension among its leading members; He was so weakened by the disease that he could barely stand up. Enervate and sap both mean to lessen the vitality or strength of; both are applied only to people. Sap implies a gradual or insidious loss of strength; enervate focuses on a loss of vitality and a general lassitude, as after an illness, during a spell of hot weather or because of a loss of moral fibre.

• The emotional strain of attending to his dying mother sapped all his strength; After his illness, he felt enervated and listless; The youth was enervated by dissipation.

Enfeeble is like enervate and sap in being used chiefly of human beings. It differs, though, in indicating an extreme weakening, as through long, debilitating illness or serious deprivation, and in suggesting a resultant helplessness: enfeebled by the insidious spread of cancer; prisoners of war enfeebled by chronic undernourishment.

Exhaust means to use up and to empty utterly, while deplete means to lessen the resources of a thing. To deplete one’s store of ammunition is to lower it to the point of danger; to exhaust it is to have none left. Deplete is often used of natural resources to express alarm at the reckless or wasteful use of unrecoverable materials: a bill that would prevent our shrinking preserve of virgin timberland from being further depleted . Where deplete refers to quantities, exhaust may also refer to qualities that cannot be measured. Spend is close to exhaust , but shares with sap the connotation of gradual exhaustion, a using up by degrees.

• The candidate’s energy was spent after the long and arduous campaign.

Spend and exhaust may apply to anything giving power.

• The fuel was spent (or exhausted ).

Sap and enervate , on the other hand, are always associated with life or with the life-giving spirit, and cannot be used of non-vital processes.

• The constant battle to protect his reputation from the envious taunts of petty people sapped his strength and enervated his spirit.