noun, plural sons of guns. Slang.
rogue; rascal; scoundrel: That son of a gun still owes me $20.
a tiresome or disagreeable matter, chore, etc.
(used as an affectionate greeting, term of address, etc.): Charlie Humpelmeyer, you old son of a gun, how are you?
(used as an exclamation of irritation, surprise, dismay, etc.).
perhaps orig. the illegitimate offspring of a soldier, though later influenced by British argot gun
thief; cf. gun moll