Sell somebody down the river
Definition 1 of 1
Pro Tip 1/3Historical Origin
Originates from when slaves were betrayed by being sold and sent down the Mississippi River. 
Despite its usage, remember 'selling someone down the river' comes from a painful historical context.
Pro Tip 2/3Strong Betrayal
It implies a deep and personal betrayal, as it impacts trust significantly. 
You really sold me down the river when you exposed my secrets to everyone.
Pro Tip 3/3Negative Connotation
This phrase is used negatively and can hurt someone's feelings if used directly. 
He felt as though his partner had sold him down the river during the negotiation.