The Collaborative International Dictionary of English v.0.48:

Prejudicial \Prej`u*di"cial\, a. [L. praejudicialis belonging to
   a preceding judgment: cf. F. pr['e]judiciel.]
   [1913 Webster]
   1. Biased, possessed, or blinded by prejudices; as, to look
      with a prejudicial eye. [Obs.] --Holyday.
      [1913 Webster]

   2. Tending to obstruct or impair; hurtful; injurious;
      disadvantageous; detrimental. --Hooker.
      [1913 Webster]

            His going away . . . was most prejudicial and most
            ruinous to the king's affairs.        --Clarendon.
      [1913 Webster] -- Prej`u*di"cial*ly, adv. --
      Prej`u*di"cial*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prejudicial
    adj 1: (sometimes followed by `to') causing harm or injury;
           "damaging to career and reputation"; "the reporter's
           coverage resulted in prejudicial publicity for the
           defendant" [syn: damaging, detrimental,
           prejudicial, prejudicious]
    2: tending to favor preconceived ideas; "the presence of
       discriminatory or prejudicial attitudes in the white
       population" [syn: prejudicial, prejudicious]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "prejudicial":
   bad, baleful, baneful, corroding, corrosive, corrupting,
   corruptive, counterproductive, damaging, deadly, deleterious,
   detrimental, differential, disadvantageous, discriminative,
   disserviceable, distressing, evil, forejudged, harmful, hurtful,
   inimical, injurious, judged beforehand, lethal, malefic,
   malevolent, malign, malignant, mischievous, noisome, noxious,
   ominous, pernicious, poisonous, preconceived, preconceptual,
   preconcluded, predecided, predetermined, predisposed,
   predispositional, prejudged, prejudging, presumed, presupposed,
   presurmised, scatheful, toxic, unadvantageous, unfavorable,
   unprofitable, unrewarding, useless, venenate, veneniferous,
   venenous, venomous, vicious, virulent, worthless