From urbandictionary.com:
"To keep nix is an old english boarding school term for keeping a lookout. It is thought to derive from the word that was shouted by said lookout when a housemaster or teacher approached. Younger boys were instructed to "keep nix" while the older boys smoked, drank, buggered each other etc etc.
"I say Jenkins, will you keep nix while I roger young perkins there!"
"Nix!"
"Quick run here comes Mr Cooke""