Stab somebody in the back
Definition 1 of 1
Pro Tip 1/3Betrayal Feelings
The phrase evokes deep feelings of hurt and betrayal as someone trusted acts against you. 
He was my best friend, and then he went and stabbed me in the back by lying to everyone about me.
Pro Tip 2/3Metaphorical Use
It's never about actual stabbing; it's a metaphor for deceit and treachery. 
I felt like she stabbed me in the back when she took credit for my idea at work.
Pro Tip 3/3Trust Element
The phrase often implies that trust existed before the betrayal took place. 
After working together for years, his decision to stab me in the back was truly unexpected.