red herring

"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 that draws attention away from the central issue, as in Talking about the new plant is a red herring to keep us from learning about downsizing plans. The herring in this expression is red and strong-smelling from being preserved by smoking. The idiom alludes to dragging a smoked herring across a trail to cover up the scent and throw off tracking dogs. [Late 1800s]