astheneō

This is the everyday medical use of the word. A person who is sick has lost the strength of a healthy body, so the same verb that means "to lack strength" is the ordinary New Testament way to say someone is ill. The Gospels use it constantly for the crowds of suffering people brought to Jesus, and the letters use it for named friends laid low by disease.

The range runs from a passing illness to being at the point of death. Lazarus, the official's son, Dorcas, Trophimus, and Epaphroditus are all described with this word, and in several of those cases the sickness is serious enough that death is near or has already come. So the verb does not by itself say how grave the illness is; the surrounding story supplies that.

When Jesus tells his followers to "heal the sick," or when the letter of James asks whether anyone is sick, this bodily sense is plainly in view. The cure or the prayer answers a physical condition.

Outside the sickbed, the same verb describes any kind of weakness that is not bodily. A person can be weak in faith, weak in conscience, weak in social or financial standing, or simply without the power to do something. The thread that ties these together is the plain idea of lacking strength, now applied to the inner life or to one's place in the world rather than to the body.

Paul uses this sense more than anyone. He speaks of the believer who is weak in faith and so eats only vegetables, and of the weak conscience that a careless stronger believer can wound. He also turns the word on himself, boasting that he is weak so that Christ's power may rest on him, and describing Christ himself as crucified in weakness yet alive by God's power.

The weakness is not always a fault. Sometimes it is a tender condition to be protected, as with the weak brother. Sometimes it is the human limitation through which God's strength is displayed, as in Paul's hardships. The reader has to watch the context to see whether the weakness is being corrected, defended, or even embraced.