sell down the river, to

sell down the river

Betray, as in They kept the merger a secret until the last minute, so the employees who were laid off felt they'd been sold down the river . This expression, dating from the mid-1800s, alludes to slaves being sold down the Mississippi River to work as laborers on cotton plantations. Its figurative use dates from the late 1800s.
See also: down, river, sell

sell down the river

Informal
To betray the trust or faith of.
See also: down, river, sell

sell down the river, to

To betray. This term arose in the mid-nineteenth-century United States and referred to selling slaves down the Mississippi River, where they would almost certainly be worked to death in the cotton fields. The term was used in its literal sense by Harriet Beecher Stowe in her best-selling novel Uncle Tom’s Cabin, but by the late nineteenth century it was being used figuratively. P. G. Wodehouse used it in Small Bachelor (1927): “When Sigisbee Waddington married for the second time, he to all intents and purposes sold himself down the river.”
See also: down, sell