His excuse sounded unconvincing, and nobody believed it.

unconvincing 是由前缀 un-（表示"不"）和形容词 convincing（令人信服的）组成的形容词。convincing 来自动词 convince（说服），加上 -ing 变成形容词。 unconvincing 意思是"不令人信服的、难以使人相信的"。