The phrase 'sold down the river' means you've been betrayed. It used to mean something far worse.
"River" was a literal reference to the Mississippi or Ohio rivers. For much of the first half of the nineteenth century, Louisville, Ky., was one of the largest slave trading marketplaces in the country. Slaves would be taken to Louisville to be "sold down the river" and transported to the cotton plantations in states further south.
In his 2010 history of the Mississippi River, journalist Lee Sandlin said that "the threat of being 'sold down the river' was seen as tantamount to a death sentence."