implacable
/implaˈkable/ · B2 · adj
implacable
🪨 im- (not) + placable (pleasable) = cannot be pleased or calmed
From Latin placare, 'to calm, soothe', the same root as placate and placid. Add the negative prefix im- and you get a person or force that cannot be soothed: implacable. English shares the word with the same meaning.