The Collaborative International Dictionary of English v.0.48:

Disadvantageous \Dis*ad`van*ta"geous\, a. [Cf. F.
   d['e]savantageux.]
   Attended with disadvantage; unfavorable to success or
   prosperity; inconvenient; prejudicial; -- opposed to
   advantageous; as, the situation of an army is
   disadvantageous for attack or defense.
   [1913 Webster]

         Even in the disadvantageous position in which he had
         been placed, he gave clear indications of future
         excellence.                              --Prescott.
   -- Dis*ad`van*ta"geous*ly, adv. --
   Dis*ad`van*ta"geous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disadvantageous
    adj 1: constituting a disadvantage [ant: advantageous]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "disadvantageous":
   baleful, baneful, corroding, corrosive, corrupting, corruptive,
   counterproductive, damaging, deadly, deleterious, depreciative,
   depreciatory, detracting, detrimental, disparaging, disserviceable,
   distressing, harmful, hurtful, injurious, lethal, malefic,
   malevolent, malign, malignant, mischievous, noisome, noxious,
   ominous, pejorative, pernicious, poisonous, prejudicial, scatheful,
   slighting, toxic, unadvantageous, uncomplimentary, unfavorable,
   unprofitable, unrewarding, useless, venenate, veneniferous,
   venenous, venomous, vicious, virulent, worthless