The Collaborative International Dictionary of English v.0.48:

Implacable \Im*pla"ca*ble\, a. [L. implacabilis; pref. im- not +
   placabilis: cf. F. implacable. See Placable.]
   1. Not placable; not to be appeased; incapable of being
      pacified; inexorable; as, an implacable prince.
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            I see thou art implacable.            --Milton.
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            An object of implacable enmity.       --Macaulay.
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   2. Incapable of being relieved or assuaged; inextinguishable.
      [R.]
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            O! how I burn with implacable fire.   --Spenser.
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            Which wrought them pain
            Implacable, and many a dolorous groan. --Milton.

   Syn: Unappeasable; inexorable; irreconcilable; unrelenting;
        relentless; unyielding.
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WordNet (r) 3.0 (2006):

implacable
    adj 1: incapable of being placated; "an implacable enemy" [ant:
           placable]