Sexy Back is not a trope. Or maybe it's a supertrope. In any case, you probably wanted one of the following:
- Sexy Backless Outfit: Backless outfit for Fanservice.
- Toplessness from the Back: A topless or naked person viewed from behind.
If you have a moment, feel free to edit the link that brought you here to point to whichever thing you were actually looking for.