1 : of, relating to, or characterized by lethargy : sluggish
2 : indifferent, apathetic
The word LETHARGIC has one of those outlandish histories, one that surprises you in the same way you might be surprised finding out one of your friends is secretly a Hare Krishna. That sort of information makes you think back to all of your interactions, trying to find clues. You end up sitting in a room, by yourself, saying,
Huh. I guess that sort of makes sense.
Anyway, LETHARGIC comes from the Greek Lethe, which in mythology is the "River of Unmindfulness." According to legend, when someone died he would drink from the River Lethe in order to forget the events of his past life. The act of forgetting of course had its side effects, the most prevalent being sluggishness, inactivity, or indifference. Hence, LETHARGY.
Contemporarily, LETHARGY is a symptom of various lifestyles and disorders. Some of note: lack of fluids, overdose of heavy foods, oversleep, kidney failure, jaundice, hepatitis, bipolar disorder, depression, and menopause.
What intrigues me about this history is that LETHARGY serves as a result of a conscious act of forgetfulness. It almost seems like a punishment, a side effect of rebounding into the afterlife. Seriously though, does active forgetting have side effects? If so, I feel like they would lean more toward symptoms like constipation or ulcers—something resulting from the act of repression. But that could be the influence of my post-Freudian conceptualizing of the act. Psychoanalysis did not exist in ancient Greece. Maybe they just forgot. And it made them tired and disinterested. It's possible.