son of a gun, you

   An approving expression used mainly by men to men, though Carson McCullers, in The Heart is a Lonely Hunter, has ‘you old son of a gun’ used by a girl to her brother.

   The term is said to derive from a child born to a serving soldier, normally illegitimate, according to Eric Partridge, A Dictionary of Historical Slang. Brewer’s Dictionary of Phrase and Fable prefers to link it with sailors, at a time when women were allowed to live on board Royal Navy ships. If the paternity of a child born on board was uncertain, it was entered in the log as ‘son of a gun’. The Partridge explanation is the more likely: either way it is clear that this expression once meant ‘bastard’.

   That original meaning has long been forgotten. ‘Bastard’ is itself often used approvingly by intimates, as are expressioe expressions like ‘you son of a bitch’, so the change from insult to compliment (‘son of a gun’ now meaning something like ‘you amazing fellow’) is understandable.