implacable

unable to be appeased

Roots

-able / -ible: able to be, fit to be, or worthy of
im- / in- (not): not or without
plac: please, soothe, settle into calm

Other words with the plac root

complacent: self-satisfied and unaware of risk
complaisant: eager to please or agree with others
placate: to calm someone down
placatory: intended to soothe, calm, or appease
placebo: inactive treatment that may still produce a perceived benefit
placid: calm, peaceful, and undisturbed
placidity: calmness or peaceful stillness

Implacable is plac with the door locked: not able to be soothed, softened, or talked down.

The committee was implacable; even a perfect explanation could not soften its decision.

Implacable is built from the negative force of im- plus the plac family. If placate is the attempt to calm, implacable is what refuses to be calmed. Think of an enemy, a rule, or a resentment that will not take the peace offering.