RedGage

 There are two different opinions on this one, but I think the one I'll submit below is more likely and believable. The term "son of a gun" dates back to the British Royal Navy.  Though officially frowned upon, the practice of having wives or prostitutes on ships was allowed from time to time.  Inevitably, these women would get pregnant and the paternity would be in question.  The original phrase was documented in ship registries as "son of a military man" (to denote that a child had been born, but the father was yet to be determined).  Eventually it evolved into "son of a gun".  This term was first referenced in writing around 1823.