behind (one's) back
In one's absence. The phrase typically suggests wrongdoing or deceit. I can't believe you were gossiping about me behind my back! Tom will be upset that we already made the decision behind his back.
behind somebody’s ˈback
without somebody knowing, especially because they would not like it: I feel guilty about going behind his back and complaining to the boss.People were often very rude about her behind her back. OPPOSITE: to somebody’s face