a shot across the bow (no S) is a warning to stop doing something. It comes from the navy, where they would fire just in front of an enemy ship (the pointy front part is called the bow) to slow or stop it.

This is a warning. If a warship wanted another ship to stop and the other ship continued on, despite visual warnings, the warship would fire a warning shot from a cannon in front of the other ship (across the bows, the front of the ship). It was a final warning to stop before the next shot was aimed at the ship itself.