forlorn hope

noun

1. a group of soldiers detached from the main group for a very dangerous mission
2. a desperate undertaking; enterprise with very little chance of success

3. Etymology: through confusion with hope

   a faint hope

Etymology: altered < Du verloren hoop, lit., lost group < verloren, pp. of verliezen, to lose (akin to forlorn) + hoop, a band, group; akin to heap