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

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