Sell Down The River Meaning

Example : 1935, "Great Britain: Vampire's Caress," Time, 23 Dec.:  [T]he Prime Minister was listened to with respect when he replied to Opposition hints that Ethiopia was being sold down the river because Britain was afraid she or her ships might suddenly be attacked by Italian airmen.2005, Douglas J. Preston and Lincoln Child, Brimstone, ISBN 9780446612753, p. 262:  "But screw it, this bastard is selling America down the river. He's a traitor."2006, Joe Nocera, "The Anguish Of Being An Analyst," New York Times, 4 March (retrieved 17 June 2009):  As a result, analysts were routinely selling investors down the river by promoting stocks purely to land banking business from companies.