The phrase "son of a gun" is an idiomatic expression that means "a young man," or more specifically, "a young and inexperienced person." The origin of this phrase comes from the idea that sailors would often use cannon balls as ballast on their ships. When they were done with their voyage, they would take the remaining cannonballs and melt them down to make musket balls. However, sometimes a cannon ball could be overlooked during inspection and end up being used in battle. If such an accident happened, it was said that "the son of a gun" had been fired - hence the phrase came to mean any young person who was new or inexperienced.