the sources of some phrases....

[Thomas Paikeday]

If anyone is interested in "dead as a doornail," Warren Clements has this in
today's Globe & Mail:

In a recent column, I said the expression "dead as a doornail" refers to the
fact that a doornail, if its end is properly bent after it is hammered in,
cannot easily be removed and reused. Marion Graham offers another version. . . .
A doornail was one used to fasten hinges to doors before screws became common.
Nails for this use were given very little temper so that they were less brittle
and would last longer under the strains of opening and closing heavy doors.
"Dead" was a blacksmith's word for little or no temper.