implacable

 
Pronunciation: /ɪmˈplakəb(ə)l/

adjective

  • unable to be appeased or placated: he was an implacable enemy of Ted’s
  • unable to be stopped; relentless: the implacable advance of the enemy

Derivatives

implacability

Pronunciation: /-ˈbɪlɪti/
noun

implacably

adverb

Origin:

late Middle English: from Latin implacabilis, from in- 'not' + placabilis (see placable)