Achilles' heel 唯一致命的弱点；弱点；致命弱点

【例句】
Peter should have a good chance of winning but his Achilles’ heel is his carelessness.
彼得本来有很 好的机会取胜，但他的致命弱点就是太粗心大意。

English is his Achilles heel.
英语是他的软肋。