blue canary
 n.— «As miners once used caged canaries to warn them of poisonous gas underground, the greatest fear of these two streetwise veterans is a “blue canary,” a police officer or emergency worker who unknowingly walks into his or her demise.» —“Homeland Security In St. Louis” by Phillip O’Connor St. Louis Post-Dispatch (Missouri) Mar. 23, 2003. (source: Double-Tongued Dictionary)

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