Behind somebody’s back
Definition 1 of 1
Pro Tip 1/3Negative Connotation
The phrase often implies betrayal or disrespect and can evoke strong negative feelings. 
His colleagues made jokes about him behind his back.
Pro Tip 2/3Secretive Actions
Used to describe actions done in secrecy to avoid another's knowledge or consent. 
He sold her jewelry behind her back to pay off his debts.
Pro Tip 3/3Trust Issues
Can indicate a breach of trust within relationships, suggesting disloyalty. 
She found out that her partner was emailing his ex behind her back.