(1) Lazily sluggish. (2) Indifferent or apathetic.
Example:
Once again the long Sunday dinner had left most of the family feeling stuffed and lethargic.
The Greek philosopher Plato wrote that before a dead person could leave Hades to begin a new life, he or she had to drink from the river Lethe, whose name means “forgetfulness” in Greek. One would thereby forget all aspects of one’s former life and the time spent in Hades (usually pretty awful, according to Plato). But our word lethargic and the related noun lethargy usually refer not to forgetting but rather to the weak, ghostly state of those who have drunk from Lethe as dead spirits -- so weak that they may require a drink of blood before they can even speak.