(as) dead as a mackerel

1. Unequivocally dead. A: "Is the squirrel dead? Poke it with a stick and check." B: "Yep, dead as a mackerel." A: "Are you sure the goldfish is dead?" B: "Oh yeah—dead as a mackerel." Any animal unlucky enough to run out into this busy street will end up dead as a mackerel.

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