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Adjective: implacable  im'pla-ku-bul or im'pley-ku-bul or im'pla-ku-bul
  1. Incapable of being placated
    "an implacable enemy"

See also: grim, inexorable, merciless, relentless, stern, unappeasable, unforgiving, unmerciful, unmitigable, unrelenting

Antonym: placable

Encyclopedia: Implacable