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.
      [1913 Webster]

            I see thou art implacable.            --Milton.
      [1913 Webster]

            An object of implacable enmity.       --Macaulay.
      [1913 Webster]

   2. Incapable of being relieved or assuaged; inextinguishable.
      [R.]
      [1913 Webster]

            O! how I burn with implacable fire.   --Spenser.
      [1913 Webster]

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

   Syn: Unappeasable; inexorable; irreconcilable; unrelenting;
        relentless; unyielding.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "implacable":
   adamant, adamantine, avenging, cast-iron, cruel, dour, firm,
   flinty, grim, grudgeful, hard, hard-core, immovable, immutable,
   impliable, inelastic, inexorable, inflexible, intractable,
   intransigent, iron, irreconcilable, merciless, mortal, pitiless,
   punitive, punitory, rancorous, relentless, retaliatory, revanchist,
   revengeful, rigid, rigorous, rock-ribbed, ruthless, steely, stern,
   stiff, unaffected, unalterable, unappeasable, unbending,
   unchangeable, uncompassionate, uncompromising, unflinching,
   unforgiving, ungiving, unmoved, unrelenting, unsympathetic,
   unyielding, vengeful, vindicatory, vindictive