Answer:
The Lethe was one of the five rivers of Hades. The word itself means oblivion, forgetfulness or concealment, as anyone who drank from the river would forget everything. Souls going to the Elysian Fields would drink from the Lethe before entering so they would have no memory of the pain of their former lives.
The word "lethal" as we know it derives from the idea of the dead drinking from hydor lethe or Water of Oblivion. The Romans associated Lethe with death because of this.