unconvincing

His excuse for being late was highly unconvincing, and no one believed him.

(Chinese) 他迟到的借口非常苍白无力，没有人相信他。

The actor gave an unconvincing performance as a seasoned military commander.

(Chinese) 这位演员饰演一名经验丰富的军事指挥官，其表演缺乏说服力。