YOU SCRATCH MY BACK ORIGIN

YOU SCRATCH MY BACK AND I’LL SCRATCH YOURS
Originally this meant nothing like the benevolent offer it now represents. The back scratching in question was delivered via the Cat o Nine Tails. Basically it was said by whoever was about to be whipped to the guy who had been given the job of doing the whipping. It basically meant that if the roles were reversed at any time in the future he would be guaranteed the same level of damage.