Red herring is nothing more than herring that has been cured by smoke, a process that changes the color of the flesh to a reddish hue.
The herring is intended to be eaten after such curing, but dog trainers learned long ago that red herring had a peculiarly persistent odor and was very useful, if trailed over the ground, for training a dog to follow a scent.
The author of The Gentlemen’s Recreation, in 1686, advised that if a dog could not be trained by dragging a dead cat or dead fox, a red herring, having a more powerful odor, could be employed, and it could not fail to serve the desired purpose.
But that which leaves so strong a scent can be used for bad purposes as well as good. A dog that gets a good whiff of red herring will lose any other scent that it has been following. Criminals who have been chased by bloodhounds have used that knowledge to advantage.
So when our small son, trying to divert our attention from the pink stickiness on his cheek, shows us the daub of jam on the cat’s back, we accuse him of dragging a red herring over the track, figuratively, by trying to turn our suspicions aside.