The definition given in the EverybodysCyclopedia entry above is somewhat late - originally (first recorded usages in English come from the late 16th century) a 'forlorn hope' referred to any small military vanguard, especially skirmishers, who where deemed unlikely to survive their commission. It was in sufficently common parlance during the English revolution to be frequently abbreviated to the mere "forlorn", for example Cromwell in a letter to a contemporary "Captain Ireton with a forlorn of Colonel Rich's regiment".
The etymology reveals what seems to me a pathetic wordplay typical of the English at war -- we borrowed the Dutch "verloren hoop", meaning literally "lost troop" (compare with the French enfants perdu) and gave it a poetic tragedy fitting to the predicament of these men. Apparently the Dutch took the word from the German "verloren haufe - Landsknecht mercenaries would offer condemned men the chance to redeem themselves through service in such detachments where death was marginally less certain than on the gallows. Later, extension of meaning through the years and a misunderstanding of the derivation has brought us to the most common contemporary meaning of an enterprise with small chance of success.
Etymology and Cromwell citation from the O.E.D http://dictionary.oed.com