I did my nerd thing and looked up the origin.  It dates back to the Latin greeting, Hail! The English form was used as a salute of two guns or more.  Later to include the use in military assaults of guns or cannons in battle.

Example:

In the Bridgemont series, almost all of the chapters will feature a salvo of cannon in greetings to ports, friendly ships, officer boarding another ship or during the many epic battles at sea.