SON OF A GUN
"Son of a Gun" originated with sailors. When a ship was in port for an extended period of time, wives and other women were permitted to live on board with the ship's crew.
Occasionally, children would be born on board and a convenient place for the birthing to occur was most often between guns on the gun deck.
If the child's father was unknown, the child was entered in the ship's log as "Son of a gun."