blue lie

Coined by Carl Klockars referring to the blue color of American police uniforms. Klockars used the term for a lie told in order to achieve compliance without resorting to force. However, the term later became generalized to all lies told by police, and then especially to the lies told by police officers to conceal questionable behavior on the part of police force. By extension, the term came to mean any lie told to protect the reputation of one's group or organization, even when not told by a police officer.

来源：wiktionary