Investigation of this useful, albeit extremely rare, adjective for matters considered licentious, obscene or scurrilous was provoked by a message from Curt Weil, who pointed out that it appeared in Jim Meddick’s Monty comic strip on 8 July 2008. Monty criticises a man for seemingly talking to a dolphin, which Monty calls a fish. The dolphin responds and his interlocutor translates it: “He said, firstly: Dolphins are not fish. They are mammals. Then he said something rather unflattering and fescennine about primates.”
The word comes from the name of the ancient Etruscan town of Fescennia, whose location isn’t known for sure, though it was somewhere near Civita Castellana or Corchiano in the modern region of Lazio. Like many rural communities, it had a tradition of ribald and scurrilous songs that were performed at festivals such as harvest-home and weddings. These could be in the form of extempore verses that were aimed at another member of the company, who was expected to respond in kind.
The Romans took over the idea, applying it particularly to bawdy verses sung to the happy couple at their nuptials, though later the fescennine verses were cleaned up and made more urbane and sophisticated.