Implacable

Unable to be stopped.

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)).

impacable, irreconcilable, unassuageable, unplacable, unpleasable