river [ME] River comes from the same root at *rival. To sell someone down the river is to betray them, especially to benefit yourself. The expression refers to the slave-owning period of American history. It was the custom to sell troublesome slaves to owners of sugar-cane plantations on the lower Mississippi, where conditions were harsher than those in the more northerly slave-owning states. The first recorded use is in 1851 by the American writer Harriet Beecher Stowe, whose best-known work is the anti-slavery novel Uncle Tom’s Cabin (1852). The ‘betray’ sense did not emerge until much later, in the 1920s, perhaps because the subject was too sensitive to be used casually. In the USA someone who has been sent up the river is in prison. The phrase originally referred to Sing Sing prison, which is situated up the Hudson River from the city of New York.