The act of actually rubbing a person's back. NOT to be confused with back massage.
It is a ritual in which the rub-ee places a rigid palm upon the person of interest's spinal region and forgoes a series of cold, unstimulating strokes. Perhaps even resulting in some pedophile breathing and unexplainable boners. The receiver of the back rub shall then feel inclined to regret choosing such a grammatically incorrect means of demanding a back massage.
Mr Asexual: You're not a lesbian. There! I said it... You owe me one hundred back rubs.
Non-Lesbian Chick: O^O *boner-inducing back rub motion*