The Collaborative International Dictionary of English v.0.48:

Unnatural \Un*nat"u*ral\ (?; 135), a.
   Not natural; contrary, or not conforming, to the order of
   nature; being without natural traits; as, unnatural crimes.
   [1913 Webster]

   Syn: See Factitious.
        [1913 Webster] -- Un*nat"u*ral*ly, adv. --
        Un*nat"u*ral*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

unnatural
    adj 1: not in accordance with or determined by nature; contrary
           to nature; "an unnatural death"; "the child's unnatural
           interest in death" [ant: natural]
    2: not normal; not typical or usual or regular or conforming to
       a norm; "abnormal powers of concentration"; "abnormal amounts
       of rain"; "abnormal circumstances"; "an abnormal interest in
       food" [syn: abnormal, unnatural] [ant: normal]
    3: speaking or behaving in an artificial way to make an
       impression [syn: affected, unnatural] [ant: unaffected]

Moby Thesaurus II by Grady Ward, 1.0:

174 Moby Thesaurus words for "unnatural":
   Gongoresque, Gongoristic, Marinistic, aberrant, abnormal, affected,
   amorphous, anomalistic, anomalous, apocryphal, artificial, assumed,
   bastard, bent, bestial, bizarre, bogus, brummagem, callous,
   calloused, cold, cold of heart, coldblooded, coldhearted,
   colorable, colored, contrived, counterfeit, counterfeited, crank,
   crankish, cranky, crotchety, deviant, deviative, different,
   distorted, divergent, dotty, dressed up, dummy, eccentric,
   elaborate, elaborated, embellished, embroidered, erratic, ersatz,
   euphuistic, exceptional, extraordinary, factitious, fake, faked,
   false, falsified, feigned, fey, fictitious, fictive, flaky,
   flinthearted, forced, formless, freakish, funny, garbled,
   grotesque, hard, hard of heart, hardened, hardhearted, heartless,
   heteroclite, heteromorphic, histrionic, hyperelegant, idiocratic,
   idiosyncratic, illegitimate, imitation, improper, insensitive,
   insincere, irregular, junky, kinky, kooky, la-di-da, labored,
   maggoty, make-believe, man-made, maniere, mannered, mock,
   monstrous, nutty, obdurate, odd, oddball, out of character,
   outlandish, overacted, overdone, overelaborate, overelegant,
   overnice, overrefined, peculiar, perverse, perverted, phony,
   pinchbeck, precieuse, precieux, precious, pretended, pretentious,
   preternatural, pseudo, put-on, quasi, queer, quirky, restrained,
   screwball, screwy, self-conscious, self-styled, sham, shapeless,
   shoddy, simulated, singular, so-called, soi-disant, spurious,
   stagy, stiff, stilted, stonyhearted, strange, stray, straying,
   studied, subnormal, supernatural, supposititious, synthetic,
   theatrical, tin, tinsel, titivated, tribadistic, twisted,
   unaccountable, unauthentic, uncanny, uncharacteristic,
   unconventional, unexpected, unfeeling, ungenuine, unmerciful,
   unreal, unresponsive, unseemly, unusual, wacky, wandering, warped,
   weird, whimsical