In Greek and Roman mythology, Lethe was one of five rivers in the underworld, or the kingdom of the dead. Drinking from Lethe (whose name means "forgetfulness") caused the souls of the dead to forget all knowledge of their previous lives.
In his poem Ode To A Nightingale Keats uses a lot of Greek Mythology.
"Lethe" is a mythical river whose waters made the dead forget their past. The word "lethe" means forgetfulness.
In Greek mythology, Lethe is one of the several rivers of Hades: those who drank from it experienced complete forgetfulness.