The Collaborative International Dictionary of English v.0.48:

Inimical \In*im"i*cal\ (?; 277), a. [L. inimicalis, fr. inimicus
   unfriendly, hostile; pref. in- not + amicus friendly. See
   Amity.]
   [1913 Webster]
   1. Having the disposition or temper of an enemy; unfriendly;
      unfavorable; -- chiefly applied to private, as hostile is
      to public, enmity.
      [1913 Webster]

   2. Opposed in tendency, influence, or effects; antagonistic;
      inconsistent; incompatible; adverse; repugnant.
      [1913 Webster]

            We are at war with a system, which, by its essence,
            is inimical to all other governments. --Burke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inimical
    adj 1: not friendly; "an unfriendly act of aggression"; "an
           inimical critic" [syn: unfriendly, inimical]

Moby Thesaurus II by Grady Ward, 1.0:

152 Moby Thesaurus words for "inimical":
   adversary, adversative, adverse, adversive, aggressive, alien,
   antagonistic, anti, antipathetic, antithetic, antonymous,
   at cross-purposes, balancing, battling, bellicose, belligerent,
   bloodthirsty, bloody, bloody-minded, breakaway, chauvinist,
   chauvinistic, chill, chilly, clashing, cold, combative,
   compensating, competitive, con, conflicting, confronting,
   contentious, contradicting, contradictory, contradistinct,
   contrapositive, contrarious, contrary, contrasted, converse, cool,
   counter, counteractant, counteracting, counteractive,
   counterbalancing, counterpoised, countervailing, counterworking,
   cranky, cross, crotchety, dead against, detrimental, difficult,
   disaccordant, discordant, discrepant, dissentient, dissident,
   enemy, eyeball to eyeball, ferocious, fierce, fighting, fractious,
   frosty, full of fight, hard, harmful, hawkish, hostile, icy, ill,
   in opposition, incompatible, inconsistent, inhospitable, inverse,
   jingo, jingoish, jingoist, jingoistic, martial, militant,
   militaristic, military, miserable, negative, nonconformist,
   noncooperative, not easy, obstinate, obverse, offensive, opponent,
   opposed, opposing, opposite, oppositional, oppositive, oppugnant,
   overthwart, perverse, pugnacious, quarrelsome, reactionary,
   recalcitrant, refractory, renitent, repugnant, resistant, reverse,
   revolutionary, rigorous, rival, saber-rattling, sanguinary,
   sanguineous, savage, scrappy, sinister, soldierlike, soldierly,
   squared off, strained, stressful, tense, trigger-happy,
   troublesome, troublous, truculent, trying, unamiable, unamicable,
   uncooperative, uncordial, unfavorable, unfriendly, ungenial,
   unharmonious, unpacific, unpeaceable, unpeaceful, unpropitious,
   unsociable, untoward, warlike, warmongering, warring, wretched