(as) dead as a herring

Definitively, undeniably dead. Likely refers to the offensive smell that emanates from a dead herring. I poked that squirrel with a stick and, yeah, it's as dead as a herring. A: "Are you sure the goldfish is dead?" B: "Oh yeah—dead as a herring." Any animal unlucky enough to run out into this busy street will end up dead as a herring.