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eternal (adj.)[eternal 词源字典]: 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" (see eon). 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.[eternal etymology, eternal origin, 英语词源]
everlasting: early 13c., "eternal" (adj.); "eternally" (adv.); "eternity" (n.); from ever + lasting. Colloquially in mid-19c. U.S., "very, exceedingly." A verb, everlast, "to endure forever," is recorded late 14c. Related: Everlastingly.
evermore (adv.): c. 1300 as one word, "at all times; all the time; forever, eternally;" see ever + more. Replacing evermo (13c.), from Old English æfre ma.