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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" (see lose). In the Mercian hymns, Latin perditionis is glossed by Old English forlorenisse.
Sense of "forsaken, abandoned" is 1530s; that of "wretched, miserable" first recorded 1580s. A common Germanic compound (cf. Old Saxon farilosan, Old Frisian urliasa, Middle Dutch verliesen, Dutch verliezen, Old High German virliosan, German verlieren, Gothic fraliusan "to lose").
Commonly 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 is usually used incorrectly in English, and the misuse has colored the sense of forlorn. Related: Forlornly; forlornness.