When you are as sick as a dog, you are very ill and vomiting.
- Sam could not go to the party, because he was as sick as a dog.
- I have never been so ill in my life. I was sick as a dog. (= I was very ill.)
- I couldn’t enjoy the outing because I was sick as a dog.
Why do people say sick as a dog, and not sick as a horse or cat? Well, some people believe that it is because dogs eat just about everything they find. Therefore they fall ill very often.