I was trying to figure out this acnestis and think that it basically means 'not scratching oneself' from the reflexive meaning of the verb which knestis comes from. Maybe then it came to apply to the back, the difficult-to-scratch-oneself place. Then to spine as back? And cheese-grater, an implement to avoid when scratching oneself anywhere, a not-scratching-oneself implement. The last a bit of a stretch I know and some will doubt the authenticity. Great site!