Despite his repeated attempts at reconciliation, their attitude remained implacable.

implacable 是由前缀 im-（表示"不、非"）和形容词 placable（可安抚的）组成的复合词。placable 来自拉丁语 placare（安抚、平息），所以 implacable 字面意思是"不能被安抚的"，引申为"不可动摇的、无法平息的"。