Fig.: to be dead, with no chance for recovery; unusable (referring to objects); finished

Classic car my foot. You might as well junk that old lemon of yours, the engine is dead as a doornail

Altro che macchina d'epoca, ormai lo puoi anche buttare via quel vecchio catorcio. Ormai il motore è solo da buttare

The poor dog was found after two days stone dead, killed by poisoned meat

Il povero cane è stato trovato dopo due giorni, morto stecchito, ucciso da una polpetta avvelenata

This expression has been in use in England since the 14th century and was commonly used by the 16th century (Shakespeare: King Henry Vi). Nails were once hand tooled and costly and, when tearing down an old house/cabin, they would be salvaged so they could be reused in later construction. Doornails though (large-headed studs) were hammered through the door and their protruding ends were bent over to secure them, making it impossible to reuse them in the future. Hence their "deadness".