A SHOT ACROSS THE BOWS
Often used metaphorically in modern days, to fire a shot across the bows means to give someone a warning. Original it was common naval practice for a ship to fire a cannon shot across the bows of an adversary’s ship to demonstrate they were ready for battle. The first mention in print can be found in a piece from the Wisconsin Democrat dated December 1939: '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.'