implacable

What it means

1. Not able to be placated or appeased.

   2 more examples

   He is knight dubb'd with vnhatche'd Rapier, and on carpet conſideration, but he is a diuell in priuate brall, soules and bodies hath he diuorc'd three, and his incenſement at this moment is ſo implacable, that ſatisfaction can be none, but by pangs of death and ſepulcher: Hob, nob, is his word: giu't or take't.

   c. 1601–1602 (date written), William Shakespeare, “Twelfe Night, or What You Will”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC, [Act III, scene iv], page 269

   [I]n the reign of Henry the Second, a body happening, by chance, to be dug up near Glastonbury Abbey, without any symptoms of putrefaction or decay, the Welch, the descendants of the Ancient Britons, tenacious of the dignity and reputation of that illustrious hero [King Arthur], vainly supposed it could be no other than the body of their justly-boasted Pen-Dragon; and that he had been immured in that sepulchre by the spells of some powerful and implacable inchanter.

   1810, J[ohn] Stagg, “Arthur’s Cave. A Legendary Tale.”, in The Minstrel of the North: Or, Cumbrian Legends. […], London: Printed by Hamblin and Seyfang, […], for the author, and sold by J. Blacklock, […], →OCLC, page 105

   Synonyms: impacable, irreconcilable, unassuageable, unplacable, unpleasable · Antonyms: appeasable, assuageable, pacable, pacifiable, placable

2. Impossible to prevent or stop; inexorable, unrelenting, unstoppable.

   The battleships Washington and South Dakota pushed through the sea with an implacable ease.

   2011, James D. Hornfischer, “The Giants Ride”, in Neptune’s Inferno: The U.S. Navy at Guadalcanal, New York, N.Y.: Bantam Books, →ISBN; trade paperback edition, New York, N.Y.: Bantam Books, 2012, →ISBN, page 345

   Synonyms: relentless, unremitting, unyielding

3. Adamant; immovable.

   Indeed Cain hated his Brother, becauſe his own works were evil, and his Brothers righteous; and if thy Wife and Children have been offended with thee for this, they thereby ſhew themſelves to be implacable to good; and thou haſt delivered thy ſoul from their blood.

   1678, John Bunyan, The Pilgrim’s Progress from This World, to That which is to Come: […], London: […] Nath[aniel] Ponder […], →OCLC; reprinted in The Pilgrim’s Progress (The Noel Douglas Replicas), London, (Please provide a date or year), page 84

In a sentence

He is knight dubb'd with vnhatche'd Rapier, and on carpet conſideration, but he is a diuell in priuate brall, soules and bodies hath he diuorc'd three, and his incenſement at this moment is ſo implacable, that ſatisfaction can be none, but by pangs of death and ſepulcher: Hob, nob, is his word: giu't or take't.

c. 1601–1602 (date written), William Shakespeare, “Twelfe Night, or What You Will”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC, [Act III, scene iv], page 269

Where it comes from

1. Latinimplācābilis“unappeasable, implacable; irreconcilable”
2. Old Frenchimplacable“harsh, unrelenting; implacable”
3. Middle Englishimplācāble“immitigable, unappeasable”
4. Englishimplacable

From Middle English implācāble (“immitigable, unappeasable”) from Old French implacable (“harsh, unrelenting; implacable”) (modern French implacable), from Latin implācābilis (“unappeasable, implacable; irreconcilable”), from im- (variant of in- (prefix meaning ‘not’)) + plācābilis (“placable; appeasing, moderating, pacifying, propitiating; acceptable”) (from plācō (“to assuage, pacify, placate; to appease; to reconcile”) + -bilis (suffix forming adjectives indicating a capacity or worth of being acted upon)).