Lethean [lɪˈθiən] a.
1.) Pertaining to the river Lethe; hence, pertaining to or causing oblivion or forgetfulness of the past.
Etymology: from Latin Lēthæ-us (adoption of Greek ληθαῖος, from λήθη Lethe, one of the five rivers of the Greek underworld) + -an.
"From the slain Victims pour the streaming Blood,
And leave their Bodies in the shady Wood:
Nine Mornings thence, Lethean Poppy bring,
T' appease the Manes of the Poets King:
And to propitiate his offended Bride,
A fatted Calf, and a black Ewe provide:
This finish'd, to the former Woods repair."
(Georgics by Virgil, John Dryden (trans.), 1697)