"Unfaithful Dogs"?

There is some kind of armed conflict going on in a part of the world called the Middle East. (This part of the world is not to be confused with Middle Earth of the English author J.R.R. Tolkien's books.) In this context, I have been told that some humans in this part of the world use to insult other humans by calling them "unfaithful dogs". I have trouble believing this. I mean, any human who knows dogs knows that there is noone who is more faithful than a dog. Such an insult would thus be very weird, since it has got nothing to do with reality. Oh well, the world is a strange place, so maybe the story is true after all.