Lethe, in Greek mythology, was the river of forgetfulness in Hades;

As I recall, one drinks from the Lethe, to forget the world of the living, just before crossing the better-known Styx, with a coin in the mouth. What I don't get is how "lethargic," which comes from "Lethe," ended up meaning sluggish or sleepy, rather than forgetful. In the myths, does drinking from the Lethe make one drowsy? Or has the word just transmogrified since the days of Orpheous?

The neighboring stream to the Lethe was the Mnemosyne, the river of memory - which is where we get mnemonic - which seems much truer to its source.