blue wall of silence

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic Strict secretiveness maintained by the members of a police force with respect to information which might be contrary to their interests, especially information concerning questionable police actions.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the secrecy of police officers who lie or look the other way to protect other police officers