The Collaborative International Dictionary of English v.0.48:

Unfavorable \Un*fa"vor*a*ble\, a.
   Not favorable; not propitious; adverse; contrary;
   discouraging. -- Un*fa"vor*a*ble*ness, n. --
   Un*fa"vor*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unfavorable
    adj 1: not encouraging or approving or pleasing; "unfavorable
           conditions"; "an unfavorable comparison"; "unfavorable
           comments", "unfavorable impression" [syn: unfavorable,
           unfavourable] [ant: favorable, favourable]
    2: (of winds or weather) tending to hinder or oppose;
       "unfavorable winds" [syn: unfavorable, unfavourable]
       [ant: favorable, favourable]
    3: not favorable; "made an unfavorable impression"; "unfavorable
       reviews" [syn: unfavorable, unfavourable]

Moby Thesaurus II by Grady Ward, 1.0:

152 Moby Thesaurus words for "unfavorable":
   adversary, adversative, adverse, against, alien, antagonistic,
   anti, antipathetic, antithetic, apocalyptic, bad, baleful, baneful,
   black, bodeful, boding, clashing, competitive, con, conflicting,
   contradictory, contrary, counter, counteractive, cross, dark,
   deleterious, detrimental, difficult, dire, disaccordant,
   disadvantageous, disappointed, disapprobatory, disapproving,
   discontented, disenchanted, disgruntled, disillusioned, displeased,
   dissatisfied, dissentient, dissenting, disserviceable, doomful,
   dreary, enemy, evil, evil-starred, fateful, foreboding, fractious,
   gloomy, hard, harmful, hostile, ill, ill-boding, ill-considered,
   ill-fated, ill-omened, ill-seasoned, ill-starred, ill-timed,
   improper, in opposition, inappropriate, inauspicious, inconvenient,
   indignant, inexpedient, infelicitous, inimical, injurious,
   inopportune, intempestive, intrusive, irrelevant, late, low,
   lowering, mal a propos, malapropos, menacing, miserable, mistimed,
   negative, noncooperative, not easy, obstinate, of evil portent,
   off base, ominous, opponent, opposed, opposing, opposite,
   oppositional, oppositive, oppugnant, out of line, out of phase,
   out of time, overthwart, perverse, poor, portending, portentous,
   prejudicial, premature, recalcitrant, refractory, repugnant,
   rigorous, rival, sinister, somber, stressful, threatening,
   too late, too soon, troublesome, troublous, trying, turned-off,
   unadvantageous, unappreciative, unapproving, unbefitting,
   uncomplimentary, uncooperative, unfit, unfitting, unfortunate,
   unfriendly, unhandy, unhappy, unlucky, unprofitable, unpromising,
   unpropitious, unready, unrewarding, unripe, unseasonable,
   unsuitable, untimely, untoward, useless, worthless, wretched,
   wrong