Behind bars means someone or something is in jail or confined in some way. It is most often used to say someone, rather than something, is in prison or jail. Sometimes it is not used in the literal sense; for example, someone might say “I’ve been put behind bars,” to mean that he or she feels confined or trapped in a situation. The saying is used because a lot of jails have barred walls, so the prisoner is literally behind the bars. This saying is usually not viewed as slang; it is a term used by many news reporters, educators, and other professionals.
“He was put behind bars for committing a crime,” is a literal usage of the saying. Alternatively, a person could say, “She is behind bars for the rest of her life” or “You will end up behind bars if you continue to break the law.” These usages are particularly common in the English-speaking world.
To use the expression in a figurative way, a person could say, “I am behind bars in this cubicle,” or “She put me behind bars by taking away the car keys.” In both cases, no one is actually behind the bars of a cell, but the speaker expresses his or her distress by comparing the situation to being in prison. The amount of distress is usually exaggerated to get the speaker’s point across. For example, the saying “I could eat a horse” simply means the speaker is very hungry, not that he or she finds horses appetizing or could actually eat an entire one.
In the 20th century, the saying was used as a name for many things. For example, “Behind Bars” is an album name by rapper Slick Rick, also known as Richard Walters. The song talked about being trapped in a life of drugs and alcohol. The lyrics mentioned feeling as if one is in a cell. It was released in 1994 and had some success on the music charts.
This saying can be used as an idiom because its usage is not limited to the literal meaning. Idioms come and go, and are generally considered to be part of a culture rather than a language. Like many modern sayings, the history of the saying is unknown. The saying could theoretically be as old as prison bars. In addition, until prisons no longer have bars, the saying is likely to be around for hundreds of years more.