The Collaborative International Dictionary of English v.0.48:

Unpropitious \Unpropitious\
   See propitious.

WordNet (r) 3.0 (2006):

unpropitious
    adj 1: not propitious [ant: propitious]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "unpropitious":
   adversary, adversative, adverse, alien, antagonistic, anti,
   antipathetic, antithetic, apocalyptic, bad, baleful, baneful,
   black, bodeful, boding, clashing, competitive, con, conflicting,
   contradictory, contrary, counter, cross, dark, dire, disaccordant,
   dissentient, doomful, dreary, enemy, evil, evil-starred, fateful,
   foreboding, fractious, gloomy, hostile, ill, ill-boding,
   ill-considered, ill-fated, ill-omened, ill-seasoned, ill-starred,
   ill-timed, improper, inappropriate, inauspicious, inconvenient,
   inexpedient, infelicitous, inimical, inopportune, intempestive,
   intrusive, irrelevant, late, lowering, mal a propos, malapropos,
   menacing, mistimed, negative, noncooperative, 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, portending,
   portentous, premature, recalcitrant, refractory, repugnant, rival,
   sinister, somber, threatening, too late, too soon, unbefitting,
   uncooperative, unfavorable, unfit, unfitting, unfortunate,
   unfriendly, unhandy, unhappy, unlucky, unpromising, unready,
   unripe, unseasonable, unsuitable, untimely, untoward, wrong