Source : Webster's Revised Unabridged Dictionary (1913)

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.

            I see thou art implacable.            --Milton.

            An object of implacable enmity.       --Macaulay.

   2. Incapable of ebign relieved or assuaged; inextinguishable.
      [R.]

            O! how I burn with implacable fire.   --Spenser.

            Which wrought them pain Implacable, and many a
            dolorous groan.                       --Milton.

   Syn: Unappeasable; inexorable; irreconcilable; unrelenting;
        relentless; unyielding.

Source : WordNet®

implacable
     adj : impossible to placate; "an implacable enemy" [ant: {placable}]