Denoting or relating to an attack made by one's own side that accidentally harms one's own forces.

When you eat dirt for the first time and leave your family to build a shack in the woods

Referring to when a police officer testifies against another police officer in a court of law.

Jargon used by police (blue) to indicate they are firing a weapon and that other officers should take care to avoid getting caught in the crossfire. In the 911 call transcript of the Tiger attack at the San Francisco zoo, an officer was heard announcing a "Blue on blue".