The lower part of the back where it curves in most. Generally located above where most people wear their pants. It can be a very sensual spot to touch lightly as a form of flirting, or just to move past people.
Stacy: Justin touched the small of my back and I melted inside.
Kim: I want to get a tramp stamp on my small of the back!
The small of the back is an anatomical term referring to the lower part of the back: specifically, the narrow area just above the butt where the spine curves into the waist.
he put his left hand on the small of their back and the right on the back of their head as he held them in a tight embrace.
Typically performed in crowded bar when trying to get around someone of the opposite sex. The act involves placing your hand on the small of the persons back right above their butt. Typically unneeded contact or mild groping.
Lisa, that guy just small backed me as he was walking by me.