What does you are as sick a dog mean?

"You are as sick as a dog" is a figurative expression used to describe someone who is very ill. It is a comparison of the person's condition to that of a sick dog, which is often associated with vomiting, diarrhea, and lethargy. The expression is used to convey the seriousness of the person's illness and the need for medical attention. It can also be used in a humorous or light-hearted manner to describe someone who is feeling unwell but not seriously ill.