The Collaborative International Dictionary of English v.0.48:

Incompatible \In`com*pat"i*ble\, n. (Med. & Chem.)
   An incompatible substance; esp., in pl., things which can not
   be placed or used together because of a change of chemical
   composition or of opposing medicinal qualities; as, the
   incompatibles of iron.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Incompatible \In`com*pat"i*ble\, a. [Pref. in- not + compatible:
   cf. F. incompatible.] [It was formerly sometimes written
   incompetible.]
   [1913 Webster]
   1. Not compatible; so differing as to be incapable of
      harmonious combination or coexistence; inconsistent in
      thought or being; irreconcilably disagreeing; as, persons
      of incompatible tempers; incompatible colors, desires,
      ambition.
      [1913 Webster]

            A strength and obduracy of character incompatible
            with his meek and innocent nature.    --Southey.
      [1913 Webster]

   2. (Chem.) Incapable of being together without mutual
      reaction or decomposition, as certain medicines.
      [1913 Webster]

   Incompatible terms (Logic), terms which can not be combined
      in thought.

   Syn: Inconsistent; incongruous; dissimilar; irreconcilable;
        unsuitable; disagreeing; inharmonious; discordant;
        repugnant; contradictory. See Inconsistent.
        [1913 Webster]

WordNet (r) 3.0 (2006):

incompatible
    adj 1: not compatible; "incompatible personalities";
           "incompatible colors" [ant: compatible]
    2: used especially of drugs or muscles that counteract or
       neutralize each other's effect [syn: antagonistic,
       incompatible] [ant: interactive, synergistic]
    3: not suitable to your tastes or needs; "the uncongenial
       roommates were always fighting"; "the task was uncongenial to
       one sensitive to rebuffs" [syn: uncongenial,
       incompatible] [ant: congenial]
    4: incapable of being used with or connected to other devices or
       components without modification [ant: compatible]
    5: of words so related that one contrasts with the other;
       "`rich' and `hard-up' are contrastive terms" [syn:
       contrastive, incompatible]
    6: not easy to combine harmoniously [syn: ill-sorted,
       incompatible, mismated, unsuited]
    7: not compatible with other facts [syn: discrepant,
       incompatible]
    8: not in keeping with what is correct or proper; "completely
       inappropriate behavior" [syn: inappropriate,
       incompatible, out or keeping(p), unfitting]
    9: used especially of solids or solutions; incapable of blending
       into a stable homogeneous mixture

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "incompatible":
   abnormal, absurd, anomalous, antagonistic, antipathetic,
   antithetic, assorted, at cross-purposes, at loggerheads, at odds,
   at variance, at war, autistic, bashful, chill, chilly, clashing,
   close, cold, conflicting, contradictory, contrary, contrasted,
   contrasting, cool, cranky, cross, departing, deviating, deviative,
   different, differentiated, differing, disaccordant, disagreeable,
   disagreeing, disconsonant, discordant, discrepant, discrete,
   discriminated, disharmonious, disjoined, disparate,
   disproportionate, dissident, dissimilar, dissociable, dissonant,
   distinct, distinguished, divergent, diverging, divers, diverse,
   diversified, frosty, grating, heterogeneous, hostile, icy,
   immiscible, in disagreement, inaccordant, incoherent,
   incommensurable, incommensurate, incongruous, inconsequent,
   inconsistent, inconsonant, inharmonious, inhospitable, inimical,
   insociable, irreconcilable, jangling, jarring, many, mismatched,
   mopey, mopish, morose, motley, multifarious, negative,
   nongregarious, opposed, opposite, out of accord, out of proportion,
   out of whack, oxymoronic, paradoxical, poles apart, poles asunder,
   repugnant, self-contained, self-contradictory, self-sufficient,
   separate, separated, several, snug, socially incompatible,
   strained, sullen, tense, unamiable, unamicable, unclubbable,
   uncommunicative, uncompanionable, unconformable, uncongenial,
   uncordial, unequal, unfriendly, ungenial, unharmonious, unlike,
   unsociable, unsocial, unsuited, variant, varied, variegated,
   various, varying, widely apart, worlds apart