fire a (warning) shot across someone's bow idiom formal
对…发出警告
to do something in order to warn someone that you will take strong action if that person does not change their behaviour
Airline employees have fired a warning shot across the company's bows by threatening to strike if higher pay increases are not offered. 航空公司员工已经向公司发出警告，威胁说如果不大幅度加薪就要举行罢工。