sell somebody down the river

Từ điển Oxford Advanced Learner 8th

sell sb down the ˈriver idiom
(informal)to give poor or unfair treatment to sb you have promised to help From the custom of buying and selling slaves on the plantations on the Mississippi river in America. Slaves who caused trouble for their masters could be sold to plantation owners lower down the river, where conditions would be worse.
Main entry: ↑sell_idiom