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"smoked herring" early 15c. (they turn red when cured), as opposed to white herring "fresh herring." Supposedly used by fugitives to put bloodhounds off their scent (1680s), hence metaphoric sense (1864) of "something used to divert attention from the basic issue;" earlier simply "a false lead":
Though I have not the honour of being one of those sagacious country gentlemen, who have so long vociferated for the American war, who have so long run on the red-herring scent of American taxation before they found out there was no game on foot; (etc.) [Parliamentary speech dated March 20, 1782, reprinted in "Beauties of the British Senate," London, 1786]
In argument, something designed to divert an opponent's attention from the central issue. If a herring is dragged across a trail that hounds are following, it throws them off the scent.
Something used to divert attention from the real issue or matter: All this talk of deficits is just a red herring
[1884+; fr the use of a dead red herring to confuse or test the scent of hunting dogs, found by 1686]