son' of a gun'
—pl. sons 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.).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.