Question: what is the idiomatic expression of red herring?
Answer: Brewer's Dictionary of Phrase and Fable (1981) gives the full phrase as "Drawing a red herring across the path", an idiom meaning "to divert attention from the main question by some side issue"; here, once again, a "dried, smoked and salted" herring when "drawn across a fox's path destroys the scent and sets the hounds ...