forlorn hope, n. — Green’s Dictionary of Slang

forlorn hope n.

[Du. verloren hoop, a lost troop (of soldiers); the orig. 16C use described a band of skirmishers or assault troops who were sent ahead of the main force; this mutates into a desperate band of men and thence a desperate enterprise]

1. the losers at a gaming table.

1608		Dekker Lanthorne and Candle-Light Ch. 3: They that loose, are the Forlorne Hope.
1685		New plot newly discovered 4: Now if he, or the Leader, or Forlorne Hope, or any of the rest, chances to hear of fresh water Souldiers [...] (his father being dead , and left him ten or twelve thousand pounds) [etc].
c.1698		B.E. Dict. Canting Crew n.p.: Forlorn-hope c. losing Gamesters.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].

2. lit. or fig., a gambler’s last, despairing bet; thus used fig.

c.1670		Purgatorium Hibernicum 7: Nees trotted with his forlorn hope / Where him & his blew Band to save ’em / had fleet prepared to receive ’em.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1801		‘Modern Dict.’ in Sporting Mag. May XVIII .
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1853		Delhi Sketch Bk 1 Oct. 62/2: I’m the ‘Forlorn Hope’ left here to bewail!
1858		Bell’s Life in Sydney 30 Oct. 3/4: Bridget Crowe [...] deposed that she was the folorn hope of the Crowe before the Court, (he had buried a pair of wives, and was likely to bury her).
1862		E. de la Bédollière Londres et les Anglais 314/2: forlorn hope, espérance perdue, dernier enjeu d’un joueur qui adopte la devise de Virgile: Una salus victis nullam sperare salutera!
1894		Truth (Sydney) 30 Dec. 1/6: Frank Stubly [...] after missing a forlorn hope out Mount Laydon way [...] passed in his checks.