canary bird

In slang, 'canary bird' carries two distinct historical meanings: a mistress kept by a wealthy man, and a person who has been imprisoned — a jailbird. The 'mistress' sense may draw on the idea of a caged songbird kept for pleasure, while the 'jailbird' sense parallels the metaphor of being locked up like a caged bird. Both senses are largely archaic and found primarily in 19th and early 20th-century slang dictionaries. In modern use, the term is most associated with a literal canary or the tech concept of a 'canary deployment.'