This is a humorous way of alluding to a fart that cannot be overheard when in company. It appears in James Joyce's "Portrait of the Artist of a Young Man" when a group of students are discussing various philosophical issues. One of them farts and another of the group asks him: "Did an angel speak?". It is also defined on FartNames.com as "any loud fart in church". Joyce obviously used it humorously, aware of its religious connotations.
by michaelion January 26, 2011