From Longman Dictionary of Contemporary Englishson of a gunˌson of a ˈgun noun [singular] American English spoken 1 ENJOY/LIKE DOING somethinga man you like or admire – used humorously Duke, you old son of a gun, how are you?2 ANNOYa man that you are annoyed with Somebody go tell that son of a gun we’re all waiting here.3 DIFFICULTan object that is difficult to deal with – used humorously4 → son of a gun!
Examples from the Corpus
son of a gun• Billy's a tough old son of a gun.• Old Laz is a pretentious son of a gun, but he's got a heart of gold.• Somebody go and tell that son of a gun that we're all waiting to go here.• This son of a gun had better work this time.son of a gun!son of a gun! spokenSURPRISEDused to express surprise → son of a gun 