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

1.
di belakang seseorang, tanpa pengetahuan seseorang
without someone's knowledge or without telling them
:
•
I can't believe she talked about me behind my back.
Saya tidak percaya dia bercakap tentang saya di belakang saya.
•
They made the decision behind his back.
Mereka membuat keputusan di belakangnya.