IMPLACABLE
implacable
(adjective) incapable of being placated; “an implacable enemy”
Source: WordNet® 3.1
Etymology
Adjective
implacable (comparative more implacable, superlative most implacable)
Not able to be placated or appeased.
Synonyms: impacable (obsolete), irreconcilable, unassuageable, unplacable (obsolete), unpleasable
Antonyms: appeasable, assuageable, pacable, pacifiable, placable
Impossible to prevent or stop; inexorable, unrelenting, unstoppable.
Synonyms: relentless, unremitting, unyielding
Adamant; immovable.
Source: Wiktionary
Im*pla"ca*ble, a. Etym: [L. implacabilis; pref. im- not + placabilis:
cf. F. implacable. See Placable.]
1. Not placable; not to be appeased; incapable of being pacified;
inexorable; as, an implacable prince.
I see thou art implacable. Milton.
An object of implacable enmity. Macaulay.
2. Incapable of ebign relieved or assuaged; inextinguishable. [R.]
O! how I burn with implacable fire. Spenser.
Which wrought them pain Implacable, and many a dolorous groan.
Milton.
Syn.
– Unappeasable; inexorable; irreconcilable; unrelenting;
relentless; unyielding.
Source: Webster’s Unabridged Dictionary 1913 Edition