The Collaborative International Dictionary of English v.0.48:

Unpromising \Unpromising\
   See promising.

WordNet (r) 3.0 (2006):

unpromising
    adj 1: unlikely to bring about favorable results or enjoyment;
           "faced an unpromising task"; "music for unpromising
           combinations of instruments"

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "unpromising":
   adverse, apocalyptic, bad, baleful, baneful, black, bodeful,
   boding, dark, dire, doomful, doubtable, doubtful, dreary, dubious,
   dubitable, evil, evil-starred, fateful, foreboding, gloomy,
   hardly possible, hopeless, ill, ill-boding, ill-fated, ill-omened,
   ill-starred, implausible, improbable, inauspicious, incredible,
   lowering, menacing, more than doubtful, of evil portent, ominous,
   portending, portentous, questionable, sinister, somber,
   threatening, unfavorable, unfortunate, unlikely, unlucky,
   unpropitious, untoward