She explained her lateness unconvincingly, so no one believed her.

unconvincingly 是由形容词 convincing（令人信服的）加上否定前缀 un-（表示"不"）和副词后缀 -ly 构成的副词。convincing 来自动词 convince（使信服），所以 unconvincingly 意思是"不令人信服地"。