Dead as a doornail
This is based on 2 things. Doors were often used to carry the dead and when they got to the funeral parlor, they dismantled the door and used the nails for the coffin.
Fit as a fiddle
A violin was considered extremely delicate, while a fiddle {technically both are the same instrument} is considered robust.
As poor as dirt
Poor people used to have dirt floors
As right as rain
It's meaning is similar to "fit as a fiddle". During the 19th century and on into the 1930s, there were many droughts and rain fixed the problems and made the crops grow. So rain made everything "right".
Sick as a dog
Dogs tend to be energetic, but when they get sick they really get sick and lethargic.
Brits tend to use "sick as a cat".
It's raining cats and dogs
There are several theories for this one. The most gruesome is that in bygone eras, drainage was so poor that stray animals, usually cats and dogs, would get drown in hard rains. When the rains were over, people would see all the dead cats and dogs and think they came with the rain.
Colder than a witch's tit
Supposedly, this is from the witch hunting days of old. People believed that witches had no maternal nature and therefore a baby couldn't suckle from her tit.
