A forlorn hope is a band of soldiers or other combatants chosen to take the vanguard in a military operation, such as a suicidal assault through the breach of a defended position, or the first men to climb a scaling ladder against a defended fortification, or a rearguard, to be expended to save a retreating army, where the risk of casualties is high.[1][2] Such men were volunteers motivated by the promise of reward or promotion, or men under punishment offered pardon for their offenses, if they survived.
OED
was invoked but never defined (see the help page).