Infirmity is bad health, or one particular illness or weakness of the body, especially the kind that comes with age or a long, frail life. Writers talk about the infirmities of old age: stiff joints, weak eyes, a heart that tires easily. It is a formal, slightly old-fashioned word. A doctor today is more likely to name the illness; a kind narrator may still say infirmity because it treats the weakness as something a person lives with, not a joke. Though actually, older writing can also use it for a weakness of character or will, but the body sense is the one this entry is for. It is not a rude word. It is a gentle, serious one.

病弱、體衰、病痛（偏書面，常指年老體弱）