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 (ĭm-plăk′ə-bəl, -plā′kə-)
Impossible to placate or appease: implacable foes; implacable suspicion.

[Middle English, from Old French, from Latin implācābilis : in-, not; see in-1 + plācābilis, placable; see placable.]

im·plac′a·bil′i·ty n.
im·plac′a·bly adv.
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References in classic literature ?
She said, with a grim implacability in voice and manner which made Tom almost realize that even a former slave can remember for ten minutes insults and injuries returned for compliments and flatteries received, and can also enjoy taking revenge for them when the opportunity offers:
After a few minutes' reflection, however, she continued, "I DO remember his boasting one day, at Netherfield, of the implacability of his resentments, of his having an unforgiving temper.
Newman gave a short laugh, but the old man seemed for the moment not to perceive it; he was gazing away, absently, at some metaphysical image of his implacability. "It doesn't much matter whether you forgive her or not," said Newman.
(32) With his marriage sunk in shared disappointment, he was face to face with Freud's trilogy of woes that beset the human condition: the certainty of death, the implacability of nature, and the suffering entailed in personal relations.
Understanding the history of Christianity in the Middle East--particularly in Mesopotamia (now Iraq)--is not a relaxed cultural experience, warns Filoni: it requires an approach that understands the reasons behind the dramatic history of that region and appreciates the life, culture, and testimony of faith of its Christians, and what inspires both their deep attachment to their homeland and their implacability towards their enemies.
The ongoing implacability of the EU against the UK government's desire to replace the Irish backstop with an "alternative arrangement" has weighed on Her Majesty's currency, while the UK's January construction PMI came in much worse than expected, at 50.6, down from 52.8 in the month prior.
Two of them include the unshakeable commitment of secret Jews, accompanied by a spiritual bifurcation that enables them to practice their beliefs in the shadow of the Inquisition; and the implacability of inquisitorial protocol as a means by which the tribunal imposes its version of truth.
What gave shape to all the milestones was the tenacity of her soul, the implacability of her wit, and the magic of her craft as wordsmith.
But these elements are also represented as part of the lethal elements of the characters' makeup: the Starks and others of Westeros are given to cold and implacability, the Lannisters of Casterly Rock and others are given to heat and fury.
What stayed with me, as much as Saba's fierce strength, was her father's implacability behind bars.
The ensemble cast were uniformly strong: authoritative Miklos Sebestyen, as the Old Believers' leader Dosifei, was the essence of moral implacability - the perfect foil for Robert Hayward's Khovansky, a twitching, seedy embodiment of corrupting power.