The decision to fire him was unjustified because he had done nothing wrong.

unjustified 是由前缀 un-（表示“不、非”）和形容词 justified（有正当理由的）组成的形容词。justified 来自动词 justify（证明...正当），加上后缀 -ed 变成形容词。unjustified 意思是“没有正当理由的、不合理的”。