Expressions in the English language come from varied sources, such as foreign languages. Another popular source of colorful expressions is the military. Sayings like “dogfight,” “GI,” “SOP” and “snafu” all come from the military. One popular nautical term is “shot across the bow.” Like many expressions, it has a military, literal definition, as well as a figurative one.

In the days before radar, radio and high-powered binoculars, one ship meeting another at a distance might not be able to tell the country from whence she hailed. Therefore, in the 18th century, the captain would order a “shot across the bow,” that is, a harmless cannonball lobbed across the bow of the ship. This was essentially a way to hail the ship and ask her to show her colors. If the colors were of an enemy country, the captain might then order an attack on the ship, but the shot across the bow had to be made first for it to be a legitimate engagement.

The shot across the bow continued on into modern times, although usually, it is only used after the firing ship has unsuccessfully attempted to communicate via radio. This may happen when a ship strays from international waters or shows aggression. It may more accurately be called a warning shot nowadays, since other ships do not always fire the shot across the bow.

Sometimes, a fighter plane escort will fire the warning shots, which may be at a ship or at another plane in the wrong place. In almost all instances now, the shot across the bow is preceded by attempts at radio communication and negotiation. The shot across the bow says, “We’re serious. We will shoot to kill.”

The phrase, "shot across the bow," also has a figurative definition. It usually refers to an action taken to show that someone is serious. For instance, someone may threaten a lawsuit. A letter from the person’s attorney asking for a meeting or resolution of the problem is considered a shot across the bow. It is the deed that shows intent to act in a certain way.

A negative campaign commercial against an opponent might also be considered a shot across the bow. It shows serious intent. Anything that might be considered a warning before serious action is taken may be considered a shot across the bow.