The English word "lethargy" comes from the legend of the river Lethe in Greek mythology. The river Lethe is also called the River of Oblivion or River of Forgetfulness and is one of the five rivers that flowed into Hades or the underworld of the ancient Greeks.
As the legend tells it, the dead are given a drink from the river Lethe to help them forget their life on earth. This same notion of forgetfulness, lack of connection and energy came to be associated with the word "lethe" in Greek or "lethargy" in English.
The word "lethe" is also associated with the goddess of forgetfulness, the personification of mental oblivion.