Mid 16th century; earliest use found in Thomas Elyot (c1490–1546), humanist and diplomat. From un- + mortal, after classical Latin immortālisimmortal.
Definition of unmortal in US English:
unmortal
adjective(ˌ)ʌnˈmɔːtl
Not mortal; not liable or subject to death.
Origin
Mid 16th century; earliest use found in Thomas Elyot (c1490–1546), humanist and diplomat. From un- + mortal, after classical Latin immortālis immortal.