borrowed from Greek nekro-, combining form from nekrós "dead body," (in plural) "spirits of the dead in Hades," derivative with the suffix -ro-, probably originally adjectival, from the base of nékes "corpses" (word known only from the lexicographer Hesychius), going back to Indo-European *neḱ- "violent death, dead body," originally action noun from the verbal base *neḱ- "disappear, pass out of existence, perish," whence also Latin nec-, nex "violent death, killing" — more at noxious
Note: Avestan nas-, attested only in the locative plural nāšū, glossed "distress, misfortune" by Christian Bartholomae (German Not, Unglück), was formerly taken as a direct outcome of Indo-European *neḱ-, and hence cognate with Latin nex; however, the meaning of the Avestan word, and hence its etymology, is uncertain. A u-stem derivative parallel to nekrós is attested in both Greek (nékȳs "corpse") and Avestan (nas, genitive nasāuuō "corpse, dead body [of people or animals]").