lost cause

1. N-COUNT If you refer to something or someone as a lost cause, you mean that people's attempts to change or influence them have no chance of succeeding. 必败无疑的事; 无望成功的人

They do not want to expend energy in what, to them, is a lost cause. 他们不想把精力投到他们认为注定不会成功的事中去。