lost cause
noun [ C usually singular ] uk/ˌlɒst ˈkɔːz/ us/ˌlɑːst ˈkɑːz/
没有希望的人（或事）
someone or something that has no chance of succeeding
I used to try to get him to do some exercise but then decided he was a lost cause. 我曾试图让他运动运动，但后来觉得他真是无可救药。