a shot across the bows

shot across the bow(s)

A warning. It refers to a warning shot from a ship, and can take the form of words or actions. Her sharp retort was a shot across the bow, letting her boyfriend know that she would not tolerate his bad attitude.
See also: across, shot
Farlex Dictionary of Idioms.

a shot across the bows

or

a warning shot across the bows

FORMAL
COMMON A shot across the bows or a warning shot across the bows is something that someone does or says to show someone else that they are prepared to fight or compete with them, often if they continue to do what they are doing. `Bows' is pronounced with the same vowel sound as the word `how'. As a warning shot across the bows of their rivals, the company is already setting very low prices. This vote is a protest, a shot across the bows to the leadership, to show them that we're here. Note: You can also use the shorter expression a warning shot. The protest should act as a warning shot to the government. Note: People often use the verb fire before these expressions, and, less often, verbs such as deliver and send. Britain's agriculture minister departed from his prepared speech to fire a shot across Norway's bows. The electorate has sent a warning shot which our politicians must now take notice of. Note: The bows are the front part of a ship.
See also: across, bow, shot
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • a shot across somebody's bows
  • shot across the bow(s)
  • fire a shot across the bow
  • a warning shot across the bows
  • across
  • bow
  • shoot up
  • shoot up on something
  • shoot-up
  • shot up