The Collaborative International Dictionary of English v.0.48:

Lethean \Le*the"an\ (l[-e]*th[=e]"an), a. [L. Lethaeus, Gr.
   lh`qaios or lhqai^os.]
   Of or pertaining to Lethe; resembling in effect the water of
   Lethe. --Milton. Barrow.
   [1913 Webster]