This is a fundamentally bad place
to be...undesirable. Frequently used in conjunction with a form of the verb "to sell
," as in, "Those swine
sold me down the river!" This means those swine screwed me, put me in an awful spot.
This derives from the American slave trade up to the 19th century, where the Mississippi River served as a main artery of human transport among slaveholding states. New Orleans was the marketplace where new slaves were brought in from Africa and the Caribbean to be bought and sold along with American-born slaves.
Generally speaking, states further down the river were closer to the slave supply and thereby got better prices for their human property. As a brutally real result, Southern slave owners often treated their slaves more harshly, since the supply was plentiful. More punishment, harder work, worse living conditions, and a shorter life expectancy than other slaves up the river as far as, say, Kentucky.
For a Kentuckian slave, therefore, to be sold down the river meant to leave an awful life for a much worse one.