backstab

To attack someone (especially verbally) unfairly in a deceitful, underhand, or treacherous manner, especially when they're not present in the place or situation that it happens.

An act of backstabbing.

After this conversation I was shocked; he asked me to leave the car so he would stay with her alone. Everything fell on me, I felt my body heavy and my heart beating very fast. It was such a backstab that it still hurts today.