"A fly on the wall" is an English language idiom, or expression that draws meaning from words not specifically related to its intent. The phrase means that a person is able to listen and watch what is happening in a particular place while not being observed. The expression is a reference to the ability of a fly to sit on a wall and also go relatively unnoticed. This phrase is most commonly used in British and American English.
The phrase first came into use in the United States of America during the 1920s. The earliest documented instance of "a fly on the wall" appeared in a February 1921 issue of The Oakland Tribune. The phrase in the article was "I'd just love to be a fly on the wall when the Right Man comes along." It became such a popular expression that it eventually spread to the United Kingdom.
In conversation, the phrase is commonly used when a speaker is indicating a wish to listen in on a particular event. For example, “I wish I were a fly on the wall at that meeting.” It can also be used by speakers in a speculative manner, where they might wonder what they would learn if they were able to witness a certain situation unobserved.
The phrase has also been used to describe observational nonfiction films called fly on the wall documentaries, which became popular in the 1960s and 1970s. In this case, it typically means that the action is taking place while the camera is made as inconspicuous as possible so that the people being filmed will act naturally. Even if the camera is in plain view, participants have often become so accustomed to its presence that they will eventually act as if they are not being filmed.
Historically, animal imagery has been a popular part of English idiomatic expressions. Many common phrases refer to specific animal traits such as the idioms "curiosity killed the cat," "a leopard can’t change its spots," and "like a chicken with its head cut off." Others are less specifically attached to animal traits, such as the phrases "a bird in the hand is worth two in the bush," "flip the bird," "dog days of summer," and "sick as a dog."