implacable

1. Incapable of being placated (synset 301789689)
   "an implacable enemy"
   also: merciless, unmerciful - having or showing no mercy
   similar to:

   • grim, inexorable, relentless, stern, unappeasable, unforgiving, unrelenting - not to be placated or appeased or moved by entreaty
   • unmitigable - incapable of being mitigated

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