Crossword Dictionary

implacable

implacable - a 
incapable of being placated; "an implacable enemy" 

wordplays

synonyms

adamant, adamantine, bullheaded, dogged, hard, hard-nosed, hardened, hardheaded, headstrong, immovable, inconvincible, inflexible, intransigent, mulish, obdurate, obstinate, opinionated, ossified, pat, pertinacious, perverse, pigheaded, self-opinionated, self-willed, stiff-necked, stubborn, unbending, uncompromising, unrelenting, unyielding, willful (or wilful)

in a sentence

He has an implacable hatred for his political opponents.  

an implacable judge who knew in his bones that the cover-up extended to the highest levels of government

merriam-webster

etymology

"unappeasable," early 15c., from Old French implacable, from Latin implacabilis "unappeasable," from assimilated form of in- "not, opposite of" (see in- (1)) + placabilis "easily appeased" (see placate). Related: Implacably.

etymonline