A castaway is someone left alone after a ship is lost — typically washed up on an island, with little or nothing. The word is made from cast away, “thrown aside.” Stories love this figure: Robinson Crusoe is the famous one, and films still use it. It can be a noun (a castaway) or, less often, an adjective (a castaway sailor). Though actually, real people in that situation are shipwreck survivors; castaway belongs a little more to stories and headlines than to hospitals and courts. It is a person, not a thing you threw away — that would be a cast-off.