To betray, especially in a manner which causes serious difficulty for the one betrayed.
[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.
Probably from the practice in the U.S., prior to the American Civil War, of trading in slaves who were transported via the Mississippi River: :* Mark Twain (1885), chapter 42, in Huckleberry Finn: “"[...
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