Debility is a rather formal, slightly old-fashioned word for weakness in the body. Doctors and older books use it more than everyday talk. It usually means your strength has drained away — often after a long illness, a hard operation, years of not eating well, or old age. A person with debility may find it hard to walk far, climb stairs, or even get out of bed. This is not the same as feeling tired for one afternoon; it is a deeper, longer weakness that sits in the body. People sometimes stretch it to a country or a system that has grown weak, though actually the home meaning is physical. In ordinary speech most people would just say weakness, or he is very weak after the flu.