One possible origin of the idiom was that red herring were used to train dogs to track scents. This was true, but the modern meaning of a false trail may have been popularised in a news story by English journalist William Cobbett, c. 1805, in which he claimed that as a boy he used a red herring (a cured and salted herring) to mislead hounds following a trail; the story served as an extended metaphor for the London press, which had earned Cobbett's ire by publishing false news accounts regarding Napoleon. The OED has another possible earlier origin in the legacy of clergyman Jasper Mayne in 1672 when he misled a servant by leaving him "Somewhat that would make him Drink after his Death" in a large trunk. When the trunk was opened, the contents were found to be red herring. 
- A herring that is cured in smoke and brine strong enough to turn the flesh red; a type of kipper.
- 1660 March 9, Samuel Pepys; Mynors Bright, transcriber, “February 28th, 1659–1660 [Julian calendar]”, in Henry B[enjamin] Wheatley, editor, The Diary of Samuel Pepys […], volume I, London: George Bell & Sons […]; Cambridge: Deighton Bell & Co., published 1893, OCLC 1016700617, page 74:
- Up in the morning, and had some red herrings to our breakfast, while my boot-heel was a-mending, by the same token the boy left the hole as big as it was before.
- (figuratively) A clue, information, argument etc. that is or is intended to be misleading, diverting attention from the real answer or issue.
- 2019 June 8, Kitty Empire, “Madonna: Madame X review – a splendidly bizarre return to form”, in The Guardian, London:
- Colombia is a red herring, however. The songs that became Madame X actually came together during Madonna’s two years in Portugal, where she decamped in 2017 when her son David enrolled in Benfica’s football academy. Madonna absorbed the local sounds with more of a mature, simpatico rather than asset-stripping eye.
- (finance) A red herring prospectus.