"Son of a gun" is an old phrase originating from British merchant ships. The crew would spend months or potentially years at sea, those without families would visit the brothels upon return to port, but some of the more financially challenged would bring the prostitutes on board and share them with some of the other crew that would split the cost. If a prostitute became pregnant it wasn't uncommon to have the birth on board to avoid hospital bills, it would happen where there was the most room available, the gun deck. Since the father was typically unknown the birth certificate signed by the ships doctor would state, "Son of a Gun."