Not every nasty thing that is said about a person is defamatory.
The rumble and tumble of speech in a free society means that not every statement at which a person takes offense is legally actionable. In short, you can't sue over every bad thing someone says about you.
Generally, factual allegations can be defamatory. Epithets are not.
Here are a couple of examples to illustrate the point. Thief and crook. Murderer and killer. Thief and murderer are accusations of specific crimes. Crook and killer, while not flattering, are not of themselves accusations of crimes.
Falsely calling someone a thief is defamatory - usually. Calling someone a thief is a specific accusation of a crime. Stealing is a crime. A thief is one who steals. It is a statement of fact, and unless qualified by surrounding context, is defamatory per se (that is, of itself). The same is true of calling someone a rapist or a murderer.
Crook is a bit different. There is no crime of being a crook. It is derogatory, but not necessarily defamatory. Generally, it is not considered defamatory per se.
But there are a whole range of epithets that are thrown around in our modern, less-than-delicate society. They are derogatory, but not defamatory.
"Punk." "Asshole." "Bitch." "Son of a bitch." They all have the green light. It's name calling, but it's not a statement of fact. No one would take the word to mean that one is literally a rectum or a female dog.
And of course the queen mother of all epithets, the "M" dash dash dash "F" dash dash dash. It may get you punched in the nose, but it won't get you sued.
On the other hand, if you make a statement that "Joe had sex with his mother," that is a a different legal issue entirely. That leads to the fascinating case of Falwell vs. Hustler Magazine, but that is a topic for a future post.
photo credit: <a href="http://www.flickr.com/photos/bitchcakes/4869567490/">*Bitch Cakes*</a> via <a href="http://photopin.com">photopin</a> <a href="http://creativecommons.org/licenses/by-nc/2.0/">cc</a>