"son of a gun"

     [This is a fast-access FAQ excerpt.]
dates from 1708; therefore, NOT son of a "shotgun marriage", which
is only recorded from 1922.  Possibly, it means "cradled in the
gun-carriage of a ship"; allegedly, the place traditionally given to
women on board who went into labour -- the only space affording her
any privacy and without blocking a gangway -- was between two guns.
Or it may mean more simply "son of a soldier".