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.

behind someone's back

Out of one's presence or without someone's knowledge, as in Joan has a nasty way of maligning her friends behind their backs. Sir Thomas Malory used this metaphoric term in Le Morte d'Arthur (c. 1470): "To say of me wrong or shame behind my back." [Early 1300s]
See also: back, behind
The American Heritage® Dictionary of Idioms by Christine Ammer.

behind someone's back

without a person's knowledge and in an unfair or dishonourable way.
See also: back, behind
Farlex Partner Idioms Dictionary
See also:
  • behind (one's) back
  • behind back
  • behind somebody's back
  • do something behind someone's back
  • go behind (one's) back
  • go behind back
  • go behind someone's back
  • burner
  • back burner
  • backburner
References in periodicals archive
It can help when you need to brag or blow off steam or tell a secret or even talk behind someone's back. Give your diary a name, and begin entries as if you were writing her a letter: "Dear Alicia, You won't believe what happened today...." Go ahead--don't hold back.
Be your own BFF
"They were all parcelled up into a bag after the race and left behind someone's back door in Beech Grove.
Duck-tectives try quacking crime
We don't gossip behind someone's back with three or more in the conversation.
Done up like a Kipper; PM peril in poll plot
You can hide and talk behind someone's back or be up front and say: 'don't do that.' I feel the second attitude is the best one even if it sometimes goes a little bit further than you would wish.
Football: CARRY ON BRAWLING, LADS
He said: "I'll tell you one thing: whatever I say to the Press behind someone's back, I'm prepared to say to their faces.
BOXING: Don't get your fighter killed .. we're gonna knock him out cold; MOUNT McKINLEY: IRON MIKE'S PAL STACY ISSUES A CHILLING WARNING TO THE SAVARESE CAMP
But I respected him as a captain because he was forthright and gave his opinions up front rather than behind someone's back.
Rugby Union: Wacky Wales from Colin the Ice Man to the crossword wizard - Chris Wyatt introduces his pals on Planet Rugby