forlorn hope

noun3

1. •
   Any dangerous or hopeless venture.

   • These cases form the forlorn hope in surgery; all saved are snatched from nearly certain death. [From the Medico-Chirurgical Transactions.] — 1841
2. •
   (no longer used) A small troop of soldiers picked to make an advance attack, or the first attack; a storming party.

   • [Benedict] Arnold, therefore, as usual with him, led the forlorn hope, marching about one hundred yards before the main body. — 1880
3. •
   (no longer used) In the plural form forlorn hopes: the soldiers collectively making up such a troop; (by extension) a group of reckless adventurers.