go behind back
go behind someone's back
1. Lit. to move behind someone; to locate oneself at someone's back. The mugger went behind my back and put a gun to my spine. Bob went behind my back and pushed me through the opening.
2. Fig. to do something that is kept a secret from someone affected by it. I hate to go behind her back, but she makes so much trouble about things like this. Please don't try to go behind my back again!