implacable

The word "implacable" comes from the Latin word "implacabilis." The prefix "im-" in Latin is a negative prefix meaning "not," and "placabilis" means "able to be appeased or soothed." The root of "placabilis" is "placare," which means "to soothe" or "to calm." Thus, "implacabilis" in Latin literally means "not able to be soothed or appeased." The word entered the English language through Old French as "implacable," retaining the same meaning of someone or something that cannot be placated or calmed.