forlorn hope

　　希望渺茫

　　【世博精解】

　　forlorn hope means plan or undertaking that is almost certain not to succeed，

　　几乎不可能实现的计划或不可能成功的事情。

　　举例：

　　Going to there rescue in a rowing-boat is a bit of a forlorn hope.

　　乘划艇去救他们，希望不大。

　　It has another mean that is‘an advance guard of troops sent on a hazardous mission'，敢死队派往完成危险任务的一支先遣部队。