Son of a Gun, origin

1. a despicable person, usually a male. (Euphemistic for son of a bitch.) : If that son of a gun thinks he can boss me around like that, he's got another think coming.
2. n.
   old buddy. : I went to school with this son of a gun! He's my old buddy.
3. exclam.
   I am totally surprised!; I am shocked! (Usually Son of a gun!) : The thing just blew up! Son of a gun!

son of a gun

 

–noun,pluralsons of guns. Slang.

1.

rogue; rascal; scoundrel: That son of a gun still owes me $20.

2.

a tiresome or disagreeable matter, chore, etc.

3.

(used as an affectionate greeting, term of address, etc.): Charlie Humpelmeyer, you old son of a gun, how are you?

4.

(used as an exclamation of irritation, surprise, dismay, etc.).

Origin:
1700–10; perh. orig. the illegitimate offspring of a soldier, though later influenced by Brit. argot gun thief; cf. gun moll