behind someone's back
US /bɪˈhaɪnd ˈsʌm.wʌnz bæk/
UK /bɪˈhaɪnd ˈsʌm.wʌnz bæk/

1.
achter iemands rug om, zonder iemands medeweten
without someone's knowledge or without telling them
:
•
I can't believe she talked about me behind my back.
Ik kan niet geloven dat ze achter mijn rug over me sprak.
•
They made the decision behind his back.
Ze namen de beslissing achter zijn rug om.