Euphemism for “Son of a bitch!”.   Fig.: this informal colloquial term has different meanings according to the context: it may be used as an exclamation of surprise or disbelief, or even a sign of affection, or it may be used to indicate a rascal or a scoundrel

That lucky son of a gun won $ 10,000 with a single one-dollar scratchcard

Quel fortunato figlio di buona donna ha vinto € 10.000 con un solo gratta e vinci da 1 Euro

Hey Ricky! You son of a gun, where have you been all this time?

Ehi Ricky! Figlio di un cane, dove sei stato tutto questo tempo?

Son of a gun, he's been complaining for the whole semester that he couldn't study, that he wasn't ready for college, that he couldn't make it, and then he aced all his exams with top marks

Figlio di buona donna! Ha continuato a lamentarsi per tutto il semestre che non riusciva a studiare, che non era pronto per l'università, che non ce l'avrebbe mai fatta e poi supera tutti gli esami con il massimo dei voti

You son of a gun! How did you manage to win that order that everybody thought was impossible?

Sei un figlio di buona donna! Ma come hai fatto ad aggiudicarti quell'ordine che si diceva impossibile?

This term is believed to have originated in the 18th century in the British Royal Navy. It applied to children born at sea, in the days when women were permitted to accompany their sailor-husbands on ocean voyages. If the father's identity was either unknown or undisclosed, the child born on board would be listed in the ship's log as "son of a gun" for the fact that women, sometimes, gave birth in the “gun deck”, the space between the ship's cannons.