The phrase “in the dock” is an idiomatic expression that means someone or something is being subjected to examination or trial. It is a British expression, more commonly used in the United Kingdom than in the United States. The origin of the phrase is rooted in the traditional layout of the English courtroom.
In the United States, the defendant and his or her legal counsel sit at a table across from the judge and to one side of the plaintiff or prosecutor. This is part of American tradition, intended to show the two sides as symbolically equal before the eyes of the law and to allow the defendant to see the witnesses against him or her. In Britain, however, the defendant sits in an enclosed area on the opposite side of the judge from the witness stand. This enclosed area, symbolically keeping the defendant in custody during the trial, is known as the dock, possibly stemming from the obsolete Flemish word docke meaning "cage." The defendant in an English criminal trial is therefore physically and literally in the dock.
From this literal basis, the expression "in the dock" has broadened in its usage to include situations metaphorically similar to being a defendant in a criminal trial. A company being audited by the government for a suspected business misdealing, a husband attempting to convince his wife that he has been faithful, or a new drug being considered for approval by Britain’s Medicines and Healthcare Regulatory Agency (MHRA) could all be metaphorically described as being in the dock.
The phrase “in the dock” is also an excellent example of the confusing difference between idiom and metaphor. An idiom is generally defined as an expression in which the meaning as a whole cannot be derived from the meanings of its component words, such as in “I’m down with that,” meaning “I agree.” A metaphor is a comparison in which one item is said to be another very different item in order to suggest a likeness, such as “That lawyer is a real shark.” Technically, the phrase “in the dock” is more a metaphor than idiom, suggesting someone’s situation is equivalent to that of a defendant on trial. For the American English speaker, who is likely unfamiliar with the courtroom definition of the word "dock," the phrase truly is an idiom in which the meaning seems unconnected to the meanings of its component words.