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1. Not to be placated or appeased or moved by entreaty
   "Russia's final hour, it seemed, approached with inexorable certainty"
   - grim, relentless, stern, unappeasable, unforgiving, unrelenting
    
2. Impervious to pleas, persuasion, requests, reason
   "Cynthia was inexorable; she would have none of him"
   - adamant, adamantine, intransigent

See also: implacable, inexorableness, inexorably, inflexible

Encyclopedia: Inexorable