The ‘bows’ in question were found at the front of a ship, essentially the ‘shoulders’ of the vessel and the phrase first appeared during the 18th century.
Sending a shot across the bows was a naval technique introduced at a time when the British navy ruled the seas and wars were fought not just on land but from the decks of mighty tall ships. The idea was not to actually fire on the ship, but to send a cannon ball near enough to signal them, force them to stop or let them know you’re ready for battle.
FLYING THE COLOURS
In the days before radio, ships were identified by their ‘colours’, or flags that represented a vessel’s purpose and nationality. So if their colours weren’t flying, another ship was legally permitted to fire a harmless warning shot across the bows in order to confirm their identity.
An early written use of the expression can be found in an 1839 edition of The Metropolitan Magazine, which read:
“In a very brief space we neared our victim, a large merchantman, whose appearance promised at once an easy conquest and a rich booty. At a signal from Stamar, a shot was fired across her bows to bring her to. She immediately hoisted a white flag, and began to take in her sails.”
The expression became commonly heard on board ships and the men working the guns were familiar with the order of firing near, not at, a ship.
A shot across the bows continued well into the 20th century, although instead of being a first measure against an unknown ship, it was used only after attempts to contact the vessel on the radio proved unsuccessful.