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

1.
za czyimiś plecami, bez czyjejś wiedzy
without someone's knowledge or without telling them
:
•
I can't believe she talked about me behind my back.
Nie mogę uwierzyć, że rozmawiała o mnie za moimi plecami.
•
They made the decision behind his back.
Podjęli decyzję za jego plecami.