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.
do something behind someone's back
COMMON If someone does something behind your back, they do it secretly, to harm you. It's horrible to have people laughing at you behind your back. He's been arranging all this behind my back.