Letha \l(e)-tha\ as a girl's name is of Greek origin, and the meaning of Letha is "forgetfulness". Greek mythology: name of a river in Hades that causes the dead to forget their lives on earth.
Letha has 4 variant forms: Leitha, Leithia, Lethe and Lethia.
For more information, see also the related name Letitia.