Sick as a Dog

Etymology:

• It is an old 16th century phrase which is a simile. Dogs have been the popular animal in the world of idiom. Just recall idioms like “in the dog house”, “a dog’s life”, “and let sleeping dogs lie”, “going to the dogs” and so on. Since among all other different pets, dog is a common household pet that sometimes gets sick for eating things that it should not or avoid to eat. So the choice for dog in the idiom is quite common.