a shot across somebody's bows

(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