catchdog
n.— «Catchdog: A term guards use to describe officers who will beat up an inmate the brass wants subdued, without being given a direct order to do so and without filing paperwork. The term was previously used to describe inmates who, acting on orders from officers, beat up other inmates to keep them in line.» —“What the lingo means” by Adam C. Smith, Jo Becker St. Petersburg Times (Florida) Dec. 30, 1999. (source: Double-Tongued Dictionary)