sell down the river

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: “"[H]e ain't no slave. . . . Old Miss Watson died two months ago, and she was ashamed she ever was going to sell him down the river, and said so; and she set him free in her will."”
• Colin Woodard (2011) chapter 18, in American nations:

  "The least fortunate wound up on the sugar plantations of southern Louisiana and Mississippi, where it was sometimes profitable to work one’s slaves to death. Being “sold down the river” originally referred to slaves being sold by Appalachian people in Kentucky and Tennessee to downriver plantation owners in the Deep South.”

• Audio (General Australian):

  (file)

sell down the river (third-person singular simple present sells down the river, present participle selling down the river, simple past and past participle sold down the river)

1. (transitive, idiomatic) To betray, especially in a manner which causes serious difficulty for the one betrayed.
   • 1935 December 23, “Great Britain: Vampire's Caress”, in Time:

     [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, Lincoln Child, Brimstone, →ISBN, page 262:

     "But screw it, this bastard is selling America down the river. He's a traitor."

   • 2006 March 4, Joe Nocera, “The Anguish Of Being An Analyst”, in New York Times, 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.

• “sell down the river”, in OneLook Dictionary Search.