Neither hide (meaning 'skin') nor hair has been around as a phrase since the 1300s to mean 'wholly/entirely'. So, if someone says, "I've seen neither hide nor hair of him", he just means he hasn't seen anything of that person at all.
I think I once heard it was an old hunting term. Animals with hide used to provide clothing, and those with hair, food. When I hunter had had a fruitless quest, he would say he "hadn't seen hide nor hair".