in bad shape

  A person who is in bad shape is in poor physical condition.

  "I really am in bad shape.  I need to get some exercise."

get yourself back into shape

  To get yourself back into shape, you need to take exercise in order to become 

  fit and healthy again.

  "Se decided she'd have to get back into shape before looking for a new job."

bag of bones

  To say that someone is a bag of bones means that they are extremely thin.

  "When he came home from the war he was a bag of bones."

full of beans

  A person who is full of beans is lively, active and healthy.

  "You'd never guess his age.  He's still full of beans!"

black out

  If you black out, you lose consciousness.

  "When Tony saw the needle, he blacked out."

as blind as a bat

  Someone whose vision is very poor, or who is unable to see anything, is (as) 

  blind as a bat.

  "Without his glasses the old man is as blind as a bat." 

blue around the gills (also green or pale around the gills)

  If a person looks blue around the gills, they look unwell or sick.

 "You should sit down.  You look a bit blue around the gills."

feel blue

  To feel blue means to have feelings of deep sadness or depression.

  "My old neighbour has been feeling blue since her dog died."

kick the bucket

  To kick the bucket is a light-hearted way of talking about death.

  "He will inherit when his grandfather kicks the bucket."

clean bill of health

  If a person has a clean bill of health, they have a report or certificate 

  declaring that their health is satisfactory.

  "All candidates are required to present a clean bill of health."

cast iron stomach

  If you can eat all sorts of food and drink what you like, without any indigestion, 

  discomfort or bad effects, it can be said that you have a cast iron stomach.

  I don't know how you can eat that spicy food.  You must have a cast iron stomach!

off colour

  If you are off colour, you are looking or feeling ill.

  "You look a bit off colour.  Are you feeling all right?"