The saying “dead as a herring” means very, very dead.
Any dead fish soon acquires an exceedingly ancient odor if left exposed for only a few hours, but the odor of a dead herring becomes twice as noticeable.
That is the reason herrings are used, by dragging them over a trail, in the teaching of young dogs to follow a scent.
The expression probably started as a variation of “dead as a doornail” back in the sixteenth century. Shakespeare used both.
He put the present one into the mouth of Doctor Caius, in Merry Wives of Windsor, Act II, scene 3. Jealous of the curate, Sir Hugh Evans, who also seeks the hand of Mistress Anne Page, Caius threatens to kill him in a duel, which the two parties, assembled in different fields, are not likely to have:
Caius (to his second, Jack Rugby) : Vat is de clock, Jack?
Rugby: ‘Tis past the hour, sir, that Sir Hugh promised to meet. Caius: By gar, he has save his soul, dat he is no come; he has pray his Pible well, dat he is no come: by gar, Jack Rugby, he is dead already, if he be come.
Rugby: He is wise, sir; he knew your worship would kill him if he came.
Caius: By gar, de herring is no dead so as I vill kill him.