A common phrase in the world of S&M, sexy back is often used by the Sadist to describe a masochist who is exposing a hairless and toned back that is to be either whipped or flogged. The Sadist will prefer a sexy back over a non-sexy back as the colour of the masochist's skin will appear more red at the end of the session, than it would if there was hair there. Also should the Sadist choose to dig their nails into the masochist's back there will be less interference of hair and thus more pain for the masochist to experience.
In Justin Timberlake's song entitled Sexy Back, Justin is expressing that he is bringing Sexy Back to his Mistress in hopes that she will be pleased to see him in massive pain by the time she is done with him. The song then goes on to expain the session in more detail...
You see these shackles
Baby I'm your slave
I'll let you whip me if I misbehave
It's just that no one makes me feel this way