The Collaborative International Dictionary of English v.0.48:

Unearthly \Un*earth"ly\, a.
   Not terrestrial; supernatural; preternatural; hence, weird;
   appalling; terrific; as, an unearthly sight or sound. --
   Un*earth"li*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unearthly
    adj 1: concerned with or affecting the spirit or soul; "a
           spiritual approach to life"; "spiritual fulfillment";
           "spiritual values"; "unearthly love" [syn: spiritual,
           unearthly]
    2: suggesting the operation of supernatural influences; "an
       eldritch screech"; "the three weird sisters"; "stumps...had
       uncanny shapes as of monstrous creatures"- John Galsworthy;
       "an unearthly light"; "he could hear the unearthly scream of
       some curlew piercing the din"- Henry Kingsley [syn:
       eldritch, weird, uncanny, unearthly]

Moby Thesaurus II by Grady Ward, 1.0:

189 Moby Thesaurus words for "unearthly":
   Christlike, Christly, Elysian, God-fearing, Olympian, a bit thick,
   a bit thin, abnormal, absurd, airy, angelic, arcane, asomatous,
   astral, balmy, beatific, beatified, beyond belief, bizarre,
   blessed, blue, bodiless, cadaverous, celestial, corpselike, crazy,
   creepy, curious, deadly, deathlike, deathly, deathly pale,
   decarnate, decarnated, discarnate, disembodied, doubtable,
   doubtful, dubious, dubitable, eccentric, eerie, eldritch, esoteric,
   ethereal, extramundane, extraordinary, extrasensory,
   extraterrestrial, fantastic, fey, freaked out, freaky,
   from on high, funny, ghastly, ghostlike, ghostly, glorified,
   godlike, godly, godly-minded, good, grisly, gruesome, haggard,
   hard of belief, hard to believe, heavenly, holy, holy-minded,
   hypernormal, hyperphysical, immaterial, impalpable, implausible,
   imponderable, in glory, inconceivable, incorporate, incorporeal,
   incredible, insane, insubstantial, intangible, kooky, livid, loony,
   lurid, macabre, miraculous, mortuary, mysterious, nonmaterial,
   nonphysical, not deserving belief, numinous, occult, odd, oddball,
   off, off the wall, open to doubt, open to suspicion, otherworldly,
   out, outlandish, outrageous, pale, paradisaic, paradisal,
   paradisiac, paradisic, passing belief, passing strange, peculiar,
   phantom, preposterous, preterhuman, preternatural, preternormal,
   pretersensual, problematic, psychic, psychical, pure,
   pure in heart, purehearted, quaint, queer, questionable,
   ridiculous, righteous, saintlike, saintly, seraphic, shadowy,
   silly, singular, spectral, spiritual, spiritual-minded, spookish,
   spooky, staggering belief, strange, sublime, superhuman, superior,
   supermundane, supernal, supernatural, supernormal, superphysical,
   supersensible, supersensory, supersensual, supramundane,
   supranatural, suspect, suspicious, tall, thick, thin,
   transcendental, transmundane, unbelievable, uncanny, unconvincing,
   unembodied, unextended, unfleshly, ungodly, unheard-of, unhuman,
   unimaginable, unnatural, unphysical, unreal, unreasonable,
   unsubstantial, unthinkable, unusual, unworldly, unworthy of belief,
   wacky, wan, weird, wondrous strange