Nayan palm stabbing?

Nayan throughout the show in some scenes is seen to preform this nasty palm scratching / stabbing action to himself. I would guess its to inflict pain when he sins but im not sure. What is this action called? Is this something that people used to do back in those times? For some reason i find it fascinating and i find it hard to believe that it was made up just for the show. Does anybody have any information on this self inflicting pain “action” that hes doing?