ficelle

(French)
device; James uses this word to indicate a character who is invented largely for the convenience of the author (compare Alfred Hitchcock’s ‘MacGuffin’ in his films)

(literally: a piece of string, particularly one used in to operate a marionette; or, a small stick of bread; also used in colloquialisms, which may have influenced James: ‘ficelle du métier’ = tricks of the trade and ‘connaître ficelle du métier’ = to know the ropes)