What Is 'Debility'?

Debility is a medical term used to describe someone who is not ok, but perhaps has no specific illness or malady.  People who are convalescing may be described as being in a state of debility.
Sometimes people may be described as having nervous debility or mental debility.  This may simply mean that they are exhausted and require rest and recuperation, in order to regain full health.   Their nervous system is frayed and they just need to gather their strength again.
Sometimes the term was used to describe someone who was feeble or weak, either due to lack of energy after illness or who was simply overwhelmed by things and easily upset.  It was often applied to women who may also have been described as 'highly strung'.
However, it is less used these days but may be used by doctors as a succinct way of summing up a state of exhaustion/convalescence.