Coming straight from the Greek word lethargia (ληθαργία), meaning "drowsiness, forgetfulness," lethargy is a lack of physical or mental energy, a sluggish or listless feeling. Lethargia in turn comes from lethe (λήθη), which means "forgetfulness" or "oblivion." One of the rivers in the Greek mythological underworld was called the River Lethe; anyone who drank from it would forget everything that happened to them in life.