"Merde" is not really an "epithet"... It is an expletive. Indeed, "merde" is very mild. At worst, and depending upon context, it just denotes vulgarity (but not necessarily) In the same way, another commonly used expletive, "putain", can be jocular, innocuous, or just plain vulgar. "Putain de merde !" is a nice combination. It can be used when you're really annoyed, or irritated, or you've just hit your fingers with your hammer while trying to drive a nail into the wall. "Bordel de merde" is another. "Putain de bordel de merde ! " is the next stage, but the overall connotations remain the same. It's just what you say if you have enough breath left. None of those expletives are actually obscene. Of course, you will not utter that sort of thing in official circumstances, for instance when being awarded the Lgion d'Honneur and that clumsy Prsident de la Rpublique has just pinned the decoration into your left tit.