forlorn hope

WordReference Random House Unabridged Dictionary of American English © 2025

forlorn′ hope′, 

1. a perilous or desperate enterprise.
2. a vain hope.
3. [Obs.] a group of soldiers assigned to perform some unusually dangerous service.
• folk-etymological alteration of Dutch verloren hoop literally, lost troop 1530–40

Collins Concise English Dictionary © HarperCollins Publishers::

forlorn hope noun

1. a hopeless or desperate enterprise
2. a faint hope
3. a group of soldiers assigned to an extremely dangerous duty

Etymology: 16th Century (in the obsolete sense): changed (by folk etymology) from Dutch verloren hoop lost troop, from verloren, past participle of verliezen to lose + hoop troop (literally: heap)