do something behind someone's back

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.
See also: back, behind
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

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.
See also: back, behind, something
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012
See also:
Full browser ?