When the film Eyes Without A Face - number nine on Mark Kermode's All Time Top 10 Best Films Evs - came up during a conversation on the Film Review Show, Mark described it as having a "mephitic air". Simon questioned whether such a word existed (this was in the days before man looking up something on a computer was possible). Mark insisted that it did, and that the word had its origin in Mephisto, a nickname for the demon Mephistopheles from the Faustian legend.
Many listeners wrote in to say that the word, meaning "foul odour" or "pervasive stink" - did indeed exist; Coleridge used it in darker Reflections, for example, while Gore Vidal had used it to describe the foul swamp atmosphere of 19th Century Washington. However, its origin was nothing to do with Mephisto. Thus both Good Doctors had been proved both right and wrong. This led to quite the bickering and tension, noted by travel reader Jo Sale as she entered the studio - "I don't like the atmosphere." Mark replied, "It's mephitic, isn't it?"
Mephitic then became a sort of resident word within the show over many subsequent episodes - as would bespeaks.