no a mis espaldas
It doesn't matter what other people say or think about you. If they do say things behind your back, then they are ugly as well.
I sleep on my back in Spanish is: Yo duermo de espaldas.
Devuelvaselo
Me retracto
yes probably because if she is pretty and if he can trust her to not say stuff in Spanish behind his back
Esconderse tras las sabanas.
Yo quiero joderte de detrás
we say: necesito que vuelvas
If you mean your back as in the part of your anatomy, the spanish word is "espalda" (S-paul-dah)
Atras is Spanish, and in English it means back, as in behind. Atras mio, for example, means behind me.
Devuelve eso
Asiento trasero