a shot across somebody's 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

(fire) a (warning) shot across somebody’s ˈbows

do something to warn an enemy, a competitor, etc. that you will take further action against them if necessary: The President’s speech on Friday was a shot across the bows of the banks. If they don’t change their policies, he will change the law.This expression refers to encounters between ships of hostile nations. One ship might fire a shot at another, not in order to hit it, but to warn it to move.
See also: across, bow, shot