A: It seems like Mary is really angry with her boyfriend’s off-color jokes in front of her friends.

B: Yes, her sharp retort is a shot across the bow, letting her boyfriend know that she would not tolerate his bad attitude.

Note: A shot across the bow(s) 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. The idiom derives from the naval practice of firing a cannon shot across the bows of an opponent’s ship to show them that you are prepared to do battle.