sell somebody down the river

sell somebody down the ˈriver

(informal) act very unfairly to somebody who trusts you; betray somebody you have promised to help: The workers thought that their own leaders had sold them down the river.This idiom comes from the days of slavery in the US. A slave who was sold to an owner further down the Mississippi river would experience worse conditions than in the states further north.
See also: down, river, sell, somebody