spect: derived from Latin "specere", meaning "look"
-ancy: the value of doing something
So expectancy means:
The value of looking out for.
Further, it means "the value of expecting".
It is always used in statistics, meaning "average".
Evolution of usage
The life value of looking out for in poor country is low.
The life value of expecting in poor country is low.
The life expectancy in poor country is low.