A doornail was one of the large iron studs formerly often used on doors for ornamentation or for added strength. The word occurred in various alliterative phrases (e.g. deaf as a doornail and dour as a doornail) but dead as a doornail is now the only one in common use.
RELATED IDIOMS :
as one door closes another opens
you shouldn't be discouraged by failure as other opportunities will soon present themselves – proverb
at deaths door = at death's door
so ill that you may die
1994 - S. P. - Somtow Jasmine Nights - How stupid of me to trouble her with my petty problems when she's probably at death's door!
close the door on = close the door to = shut the door on = shut the door to
exclude the opportunity for
refuse to consider
1999 - South China Morning Post - Fergie did not close the door on the couple reconciling some day.
door to door
(of a journey) from start to finish
visiting all the houses in an area to sell or publicize something
lay something at his door
regard or name someone as responsible for something
This phrase may have arisen from the practice of leaving an illegitimate baby on the doorstep of the man who was identified as its father.
leave the door open for
ensure that there is still an opportunity for something
open the door to
create an opportunity for
1995 - Kindred Spirit - By recreating the space in which you live or work, Feng Shui can open the door to abundance, wellbeing and a Renewed Sense of Purpose!
show him the door
dismiss someone unceremoniously from your presence
a toe in the door
a first chance of ultimately achieving what you want
a position from which further progress is possible – informal
The image here is of placing your foot in a doorway in such a way as to prevent the door being closed in your face.
on your doorstep = on the doorstep
close at hand
1998 - New Scientist - The solution to Underbill's problem was on his doorstep.