behind someone's back; from behind
Tom está mentindo pelas costas dele.
Tom is lying behind his back.
They laugh at him behind his back.
She stabbed him in the back.
Você não devia falar de Jack pelas costas dele.
You shouldn't talk about Jack behind his back.
Saia um pouco para podermos falar de você pelas costas.
Step out for a bit so we can talk about you behind your back.
— Questão de Honra (1992)
Tom me apunhalou pelas costas.
Tom stabbed me in the back.
Só vai dar certo se Alphonse atirar pelas costas.
It will only work if Alphonse shoots him in the back.
— A Noite Americana (1973)