I was always careful to draw my knife down on the face, never across or upwards. Always down. So that if the knife slips you don’t cut an artery. After all, chivving is chivving, but cutting an artery is usually murder. Only mugs do murder. Billy Hill. Criminal. Card Sharp. Ex-prisoner.
A shiv is for cutting, and stabbing.
Why would a nit-keeper be carrying a weapon?