mid-12c., forloren "disgraced, depraved," past participle of obsolete forlesan "be deprived of, lose, abandon," from Old English forleosan "to lose, abandon, let go; destroy, ruin," from for- "completely" + leosan "to lose" (from Proto-Germanic *lausa-, from PIE root *leu- "to loosen, divide, cut apart"). In the Mercian hymns, Latin perditionis is glossed by Old English forlorenisse. OED's examples of forlese end in 17c., but the past participle persisted. Sense of "forsaken, abandoned" is 1530s; that of "wretched, miserable" first recorded 1580s.
A common Germanic compound (cognates: Old Saxon farilosan, Old Frisian urliasa, Middle Dutch verliesen, Dutch verliezen, Old High German virliosan, German verlieren, Gothic fraliusan "to lose").
In English now often in forlorn hope (1570s), which is a partial translation of Dutch verloren hoop, in which hoop means "troop, band," literally "heap," and the sense of the whole phrase is of a suicide mission. The phrase more often than not is used in English as if it meant "a faint hope," and the misuse has colored the meaning of forlorn. Related: Forlornly; forlornness.