be (as) dead as a doornail

1. To be no longer alive, for certain. (Doornails were hammered in a such a way that they could not be reused.) I poked that squirrel with a stick and, yeah, it's as dead as a doornail. A: "Are you sure the goldfish is dead?" B: "Oh yeah—it's dead as a doornail." Any animal unlucky enough to run out into this busy street will be dead as a doornail in a matter of seconds.

2. To be totally defunct. Oh, that idea from last week's meeting is dead as a doornail now that the CEO has vetoed it. Good luck getting a refund from that company—it seems to be dead as a doornail these days. His mayoral campaign is likely dead as a doornail now, thanks to this exposé about his shady business dealings.