tarnation (n.)

c. 1300, dampnacioun, "condemnation to Hell by God," also "fact of being condemned by judicial sentence," from Old French damnation, from Latin damnationem (nominative damnatio), noun of action from past-participle stem of damnare "to doom, condemn" (see damn). As an imprecation, attested from c. 1600.

Damnation follows death in other men,

But your damn'd Poet lives and writes agen.

[Pope, letter to Henry Cromwell, 1707 or 1708]

late 14c., from Old French eternel "eternal," or directly from Late Latin aeternalis, from Latin aeternus "of an age, lasting, enduring, permanent, everlasting, endless," contraction of aeviternus "of great age," from aevum "age" (from PIE root *aiw- "vital force, life; long life, eternity").

Used since Middle English both of things or conditions without beginning or end and things with a beginning only but no end. A parallel form, Middle English eterne, is from Old French eterne (cognate with Spanish eterno), directly from Latin aeternus. Related: Eternally. The Eternal (n.) for "God" is attested from 1580s.