The Earl of Kent in Shakespeare’s King Lear (1603) refers to Oswald in Act II Scene II as,

…nothing but the composition of a knave, beggar, coward, pandar, and the son and heir of a mongrel bitch.

Hardly a compliment.  But is it defamatory?

According to Canadian law, it isn’t.

In Ralston v. Fomich Spencer J. of the B.C. Supreme Court stated,

The earliest mention of the term in the Oxford English Dictionary (Second Ed. 1989) Vol. 15, page 1008, is from circa 1330…

In my opinion the words “son of a bitch” by themselves are not capable of any defamatory meaning.  They are peculiar, in that they take their meaning either from the tone of voice used or from whatever adjective accompanies them.  They are a translucent vessel waiting to be filled with colour by their immediate qualifier.  Thus, one has sympathy for a poor son of a bitch, admiration for a brave son of a bitch, affection for a good old son of a bitch, envy for a rich son of a bitch and, perhaps incongruously, dislike for a proper son of a bitch.  Why right thinking people should dislike anything that is proper is rather a mystery unless proper is used to mean “real”, but I am confident that is the colour that adjective gives to the expression.

Some even think it’s a better alternative than other expletives. Roy Blount Jr. says in Equire,

“Son of a bitch” carries gravitas accumulated over centuries. An asshole is just an asshole, a hapless chump, a pointlessly obnoxious hindrance. An asshole can be an evil schemer, but he or she can’t be a worthy opponent. If an asshole gets over on you, you feel dumped on. If a son of a bitch beats you, you can live with it. You’ve probably learned something.

…We don’t need any more assholes. We need a son of a bitch.

h/t Duhaime’s Legal Dictionary, and a couple SOB law students that remain anonymous (for now)