The phrase “cat among the pigeons” is an idiom that usually includes "putting" or "setting" the cat there. It is used to refer to someone who takes some type of action, usually intentionally, that causes a disturbance. In many instances, it refers to a disturbance that is violent in nature or a statement or action that causes people to become exceedingly angry or alarmed.
The idiom comes from Britain and refers to the commotion that would ensue were a domestic cat to be released among captive birds. The cat would chase the birds, and the birds would become frightened. The cat would be likely to injure or harm the birds, and there would be a disturbance that could be described using another common idiom, “feathers will fly,” which refers to feathers flying through the air when a large flock of birds is suddenly disturbed or frightened.
Idioms are phrases common to a language or culture that are used to express an idea using a figurative example. The example is not meant to be taken literally, but because the person hearing the phrase is expected to know what the phrase describes, it is meant to convey a figurative description. For example, a speaker might say, “Revealing the minister’s mistress set a cat among the pigeons.” The speaker doesn’t literally mean a cat was released among the birds, but is describing the disturbance that likely resulted in the minister’s church once it was revealed that he had a mistress.
As with many idioms still in use, the phrase can be traced to a time when people lived more closely with animals and when the majority of people were involved with agriculture. The phrase likely came into common usage when keeping a dovecote, or dove house, was common in Britain. Pigeons were kept as a food source, and most people would have been familiar with the chaos that ensued if a cat got into the dovecot or if someone intentionally put a cat in the dovecot.
The phrase has lent itself to literary and musical references. Agatha Christie’s 1959 detective novel and Julia Golding's 2006 young adult novel are both titled Cat Among the Pigeons. The British pop band Bros also released a song in 2005 named for the phrase.